Scope : 

I have been inspired by @Steve87 to embark on repairing , as he facilitated  3 x faulty MAX7.2 machines  for which I am very grateful . We repaired one machine , but that was really just repairing on module level (faulty  MPPT) . The second machine has two AC side IGBT devices shorted , but the rest of the main board seems ok , it gives fault code 09 (soft start fail). The third one was as dead as a doornail and this is the one that prompted me to repair and document my progress in order to help myself and others , because already I have so much to thank to @Coulomb and @maxo for some powerful partial / full schematics and in general the documentation available on the AUS forum of Coulomb and Weber .   So this machine  I am going to use to  repair in stages , as you will see it has some spectacular nasty faults/failures . It would probably not be economically viable to repair this board , but the focus is on learning how to repair other boards. In the process it will probably take me weeks to repair this one , if it can be repaired at all , but the end goal is learning and to possibly attract more gurus and expertise on repair/support.

The journey started when I switched on the Machine 3 for the last time  ,  then smoke escaped from the machine itself . I decided to disassemble it . The Machine still looks very neat inside , I believe its not an old machine as there is no dust layers etc etc and really looks new .   game on.

 

Chapter 1a :  The Baby Face Assassin 

So I tried to check for visual burns and damage , and finally saw a power diode that had burn marks . This is the diode that feeds the ac side igbt driver transformer and is fed via the SPS 12V secondary transformer . so I reckon this could be a shorted transformer primary causing the diode to burn. So I de-soldered the diode , and switched  the main board on via the SPS module , in anticipation that I have relieved the main power supply from a short circuit.  But , instead , all hell broke loose when I switched on again  ,  devices just started to explode and stink and smoke . Something on this board was just shooting from the hip for fun to make a fool of me. So I remember that old Eagles lyrics : ' .... I had to find the passage back to he place I had been before . Goodnight says the night man , you are programmed to receive , you can check out any time you like , but you can never leave ..."

The blown devices were all electrolytic capacitors on the 5V , 12V ,15V and -12V rails. So it became apparent the the SPS power supply on he main board  was a fault. Before of all of this I had removed the control board and as many removable modules , as I possibly could. Fortunately no damage o the boards themselves . Even the capacitors did not open up their shells , although the explosions at times had been horrendous. 

So I had to drill down and study the partial schematics . I redraw the sps power supply as shown below :

 

The SPS+ primary supply comes from three sources really , that is mains , battery or PV , they are wired OR together , that is not shown on the schematic. I limited my damage by introducing a variable 60V bench PSU courtesy of @Steve87. Current limited it to 500mA and set dc to 40V.   removed faulty capacitors and tried to isolate load circuits as far as i can , e.g. removing filter inductor to mosfet drive supply , cutting a track carefully to the bus soft start circuit etc.  

Switched on again , and long story short  , the SPS flyback converter around TX9 and U10 was just shoveling out as much DC on the 12V output as it received from the primary .  I measured the duty cycle of U10 and found it to be at 50% (50% by the way is the max limited by UC3845 type ). The 12V output was roughly as high as the supply DC input. So there is the cause of our explosions , 12VDC probably went as high as 50V !!!!. To limit this I merely turned down the DC input to about 16V in order to minimize my losses further. Note that the U10 chip will only bootstrap at 35V , so you must have that initially then you can ramp down again . In the future I will wire OR a separate 12V supply to the U10 VCC to bypass the bootstrapping.

So I studied the voltage closed loop feedback circuit . It basically samples the 12V output (R209/R210 on the schematic) and then controls a precision shunt regulator U9 , which in turn   controls the diode current thru opto-isolator U8 . U8 transistor is connected to the COMP input of the PWN controller , which in layman's terms will decrease duty cycle the more the COMP pin is pulled towards ground (GND) . Initially I checked if the COMP input works by shorting U9 output to ground. Indeed the PWM controller switched off. So at least some of the feedback circuitry is working . Then I discover that the VREF input of U9 is fixed at 0.00V . Which is shouldn't because of the R209,R210 divider across the 12v output . But it is just a simple divider , so I removed U9 in the hope that it has failed and pulled down the divider . Again I was made a fool of , as that divider midpoint stayed at 0V . In other words there was no feedback voltage provided to  decrease the duty cycle of the PWM . I tested both R209 and R210 . R209 showed 10K , R210 showed 10.5K in-circuit. The resistance of 12V down to GND is about 500 ohms , so this was a giveaway that R210 is open circuit. But how the hell, this is a 38.3K resistor how can it go open circuit .  Out of desperation I soldered a 39K resistor across R210 pads , with the original one still in situ . 

Switch on again , and the SPS was regulating at exactly 12.15VDC . I varied the input and even took it all the way up to 56VDC . Perfect . Short circuited the 12v output , limited perfect.

Can you believe a tiny 38.3K resistor R210 going open circuit !!!!!!!!!!!!!! . This resistor will henceforth be known as the Baby-Face Assassin.

Need to do a lot of cleanup around the SPS  before I am going to address the load circuits and repairing each section bit by bit . 

 

 Why such a big margin on the open loop transfer ? . Also there is no transzorb or voltage crowbar . I am going to breadboard a programmable crowbar around the TL431 and a triac and have it onto the 12V output as long as  am repairing the rest of the circuits. Below you can see my point , pulse width is 800ns to regulate at 12V from 56V input. That is about one fifth of 50% pulse width , which will indeed equate to around 60V with max duty cycle.

 

1 hour ago, BritishRacingGreen said:

I have been inspired by @Steve87 to embark on repairing , as he facilitated  3 x faulty MAX7.2 machines  for which I am very grateful .

Good on the both of you!

1 hour ago, BritishRacingGreen said:

Chapter 1a :  The Baby Face Assassin 

A thrilling WhoDunnit! Thanks!

1 hour ago, BritishRacingGreen said:

I redraw the sps power supply as shown below :

Excellent! Yes, Maxo didn't seem to have a great feel for how circuits should be drawn, or he just didn't have the time or inclination to do more than document what he found as he found it. I'd love to redraw it, but I don't have the original CAD package, so I'm limited to nips and tucks here and there.

I see that most if not all of the part designators (e.g. TX9) are the same as the 5 kVA schematic. Did you find that they lined up perfectly? Or just redrew Maxo's circuit and designators with better layout?

1 hour ago, BritishRacingGreen said:

and then controls a precision shunt regulator U9 , which in turn...

I think you mean U7. I believe that we all have unlimited ability these days to edit our own posts.

1 hour ago, BritishRacingGreen said:

devices just started to explode and stink and smoke . Something on this board was just shooting from the hip for fun

I think that this board will largely become a source of parts, especially transformers and multi-winding inductors, even if some have to be rewound. The lack of any crowbar protection is indeed a big omission. As you have seen, that power supply has easily enough power to do very considerable damage. I guess Voltronic consider the main board to be replaceable as a whole, but the reality for many owners is that repairing would be a good option in many cases. I get PM'd by many people who want to repair their boards, and would be economically stressed if they had to replace the whole thing.

This power supply is a case in point: the only apparent failure was an SMD resistor worth less than one US cent (in quantity).

I agree that this board can become a great learning opportunity, however.

Edit: What schematic capture software do you use? Is that Protel/Altium?

Edited October 25, 20223 yr by Coulomb

On 2022/10/25 at 9:58 AM, Coulomb said:

Edit: What schematic capture software do you use? Is that Protel/Altium?

The diagram mentions "KiCad EDA" which is open source and you can download from kicad.org.

On 2022/10/25 at 2:58 PM, Coulomb said:

I see that most if not all of the part designators (e.g. TX9) are the same as the 5 kVA schematic. Did you find that they lined up perfectly? Or just redrew Maxo's circuit and designators with better layout?

Thank you for your response, Coulomb .  One of the absolutely positives is that the main board  subsystems are merely a copy paste from the 5KW design. So that's where maxo's schematic is making life so much easier . Not only are the designs the same , the component numbering as well . I performed a lot of checks and partial tracing as well to feel confident about this claim . The Max has some differences , which i will endeavor to point out at times. For instance you will see on the =12v rail there is an additional 10000uF 16V cap called CE1 (?) . Also there is R66  and R67 (5R1) on the 15V winding output that is not shown on maxo's drawing. Could be maxo forgot it or it is introduced on max (although he numbers 66 and 67 suggest it should have been imported from 5KW design) .  And then of course there are differences between machines like extra mosfets, igbt's  parralelled  etc.

On 2022/10/25 at 2:58 PM, Coulomb said:

I think you mean U7. I believe that we all have unlimited ability these days to edit our own posts.

 

Yes U7 , sigh , I am getting old.

 

 

On 2022/10/25 at 2:58 PM, Coulomb said:

Edit: What schematic capture software do you use? Is that Protel/Altium?

Kicad an open source EDA . I am using version 6 , kicad had its quirks in the early days , but it has come good. 

Off - topic :  I am also using kicad for  non electrical diagramming . So I had a project that had some configuration intensive data for embedded controllers , which was difficult to entertain creating the images manually , and of course error prone  . So I created a visual config tool using kicad . Then create the netlist . I designed a simple recursive descent compiler to read the netlist , and then  link the binary data for my embedded controllers.  That was pure fun. 

7 hours ago, BritishRacingGreen said:

For instance you will see on the [-]12v rail there is an additional 10 000 μF 16 V cap called CE1 (?) . Also there is R66  and R67 (5R1) on the 15V winding output that is not shown on Maxo's drawing.

I'm sure that there are inaccuracies and omissions in Maxo's schematic trace, and I'm sure he would agree. I happen to have a spare main board of the same vintage as the one Maxo traced, so when it's not silly-o-clock, I can go and check this.

7 hours ago, BritishRacingGreen said:

I designed a simple recursive descent compiler to read the netlist ,

As you do... not often! Respect! 😎

Quote

That was pure fun. 

😀

Chapter 1b :  Safety Net 

I decided to add a voltage crowbar to the +12V rail after I have repaired it . The circuit I Vero boarded is shown below.  inspired by the TL431 shunt regulator that is used in the Axpert PSU , I used this device to detect my crowbar voltage threshold. The potential divider of R1 and R2 will set the U1 ref pin to 2.5V when the input voltage reaches 17.5V  . The TL431 will sink current at 2.5v and above , pulling its cathode low , which will in turn fire the triac Q1 that will clamp and latch.

I choose 17.5V as I don't have precision 1% resistors, i only have 5% , so I added margin . 

The reason for using a triac instead of scr , is so I can bias the triac in one of its 4 quadrants with a negative voltage in relation to mt1.  That's why the triac has mt1 on positive and mt2 on negative. Otherwise , if using an scr , we would require a level shifter via an additional pnp transistor and a couple of resistors.

C1 filters transients , but does add to reponse time , but I think this is good enough for my function.

 

 

I have tested the clamp and wired it onto the 12V rail of the max main board . With this in place , I have really thrown the kitchen sink at the PSU , I increased the current limit of the input to 10A , and have really ungracefully interrupted the input to check stability. The clamp never triggered , which means the metastable window during PSU startup yielded no instability issues. I would have liked to have raised the input voltage to 75V as well , but the variable bench PSU has a 56V limit.

below is an image of the temporary repair work . Note that I have not soldered the new components in the thru-holes. Also I have no TL431 SMD so I 'surface mounted' a thru-hole version .

I have no plans yet to tidy up this section. This will depend wether the rest of the board will become serviceable.

 

 

So now that we have a half-decent 12V rail , we will be  moving onto the battery side mosfet drivers , and repair and test anything there  . To be continued ...

 

Hi @Coulomb, one of our members is currently having a problem with his synapse inverter. I quote his  problem and request  below :

".Was wondering if you have any experience or could possibly help me with a 24 Volt Synapse Inverter. It operates perfectly in Off Grid Mode and it also charges the batteries perfectly when the on / off toggle switch is "Off" The problem arises when the Toggle Switch is on. The screen shows that the load is supplied in Bypass and the battery is being charged but in actual fact the batteries constantly get discharged by approximately 0.6 - 1 Amps. Was wondering if you could point me in the correct direction."

 Any pointers you might have? 

Edited October 28, 20223 yr by BritishRacingGreen

Chapter 2 : What is all this bi-directional grid-tied power chain stuff , Anyhow ?

Ok before we get to the mosfet drivers etc etc, it is worth stopping and look at the context of the power chain . There's still many things I don't understand and we will draw from the guru's by asking the right questions.  After all we must know what we are repairing ???!!!!!

There are two things that's super significant regarding the power chain :

1. The chain is bidirectional , i.e  the charge flow of inverter power and the discharge flow  is done through the same chain. wow !! Therefore there is no notion anymore of a seperate grid charging circuit.wow!!

2. The inverter system is grid-interactive , albeit this is so called off-grid inverter . The only difference between true grid-tie (eg sunsynk) and the max , is that the max is configured not to export power onto the grid , or let us say it keeps exporting to safe minimum.  Why is grid-tieing important ? It is a mechanism to seamlessly blend blend  grid power with that of inverter output power. This is magic , the notion of the term  bypass is very yesteryear . In the modern axpert the grid may bypass from no power to full power with infinite number of steps in between. hence 100% load power can get , for instance , 75% from pv and 25% from grid , if in SUB mode.

We'll start off by looking at the power chain schematic as shown below :

 

 

There are really 3 main subsystems :

1. a dc to ac converter (red portion on the right hand side)

2. a dc to dc converter that can bidirectionally converter the high dc side (VBUS) to lo battery voltage and vice versa. (blue and green section on the left hand side)

3. a buck step down converter (yellow section)

Note not shown is the pv connection , which is merely wired onto the VBUS

The VBUS is around 400VDC . This is required because when the inverter is converting DC to AC , it will require up to the peak voltage of 220V rms (square roott of 2 x rms).  The battery low voltage side sits typically at 48V and higher.  

 

Basic operation of grid charge flow :

1.  The full bridge IGBT's on the right hand side are switched off , and the bridge becomes merely a full bridge rectifier via its body diodes to convert incoming grid to dc bus voltage . @Coulomb is this correct?   The dc to dc converter will convert the VBUS high voltage to  low battery voltage . The transformer has a 1:8 ratio for this purpose . So the battery will charge from the grid. Of course , with pv connected onto the VBUS , the battery will also charge via pv, but lets forget the pv for the time being.

2. The buck converter is under processor control and this is the charge regulation mechanism to throttle the charge current and control the battery charge voltage.

 

Basic operation of discharge flow  :

1. the battery voltage is converted to high vbus voltage by means of the dc to dc converter.

2. The buck converter does nothing other than being a pass-thru to the right hand side.'

3. The processor chops the VBUS voltage in order to produce pulse width modulated igbt drive for the igbt's in the DC to AC converter on the right hand side of the schematic. The resultant bridge output is filtered from harmonics of the PWM square wave by means of LC filtered , and its output is a pure sinusoid.

 

Basic operation of grid-tie function :

Well this is rocket science still in my book. However there is no trace or evidence of analogue hardware that is dedicated to grid-tie function , other than a grid-tie relay !!! So this means the entire function is under software control, albeit this consist of very complicated algorithms.

here is what I understand so far :

1. Inverter samples the ac grid and locks its own output to voltage and phase of the grid.

2. Once locked , it operates a relay which ties the grid to the inverter output.

3. Because of the very low impedance of the grid side , the inverter will now become a current source , rather than a voltage source. It will increase is own voltage ever so slightly above that of the grid , in order to affect current flowing into the loads. In the case of true gridtie , a feedback current transformer is monitored by the inverter , in order not to increase its voltage beyond the point of the injected current flowing into the main grid. So this is the mechanism for AC coupled blending , the inverter will merely raise its own voltage to the point where no more current can be pushed because of low pv or battery. The rest comes from the grid.

 

I have assumed a lot of things here , but I know Coulomb will correct me where i am wrong.

 

 

 

 

On 2022/10/27 at 5:28 PM, BritishRacingGreen said:

For instance you will see on the +12v rail there is an additional 10 000uF 16V cap called CE1 (?) . Also there is R66  and R67 (5R1) on the 15V winding output that is not shown on maxo's drawing.

I found R67 and R66 on my spare board, so I added them to Maxo's schematic. They are 10R0 on this board, however. So I also added a note that they go to 5R1 on MAX models. So there is now nacrt10c.pdf here.

I didn't see any CE1 or other 10 000 μF capacitor near the main power transformer, so I left it off. Is it across C78, or perhaps replacing C78, on your Axpert MAX?

2 hours ago, BritishRacingGreen said:

Basic operation of grid charge flow :

1.  The full bridge IGBT's on the right hand side are switched off , and the bridge becomes merely a full bridge rectifier via its body diodes to convert incoming grid to dc bus voltage . @Coulomb is this correct?

I don't believe it is, no. If it operated that way, the diodes would conduct only during the peaks of the grid sine wave, and that's terrible power factor. We can't get away with bad power factor over a few hundred watts these days, and less than that in Europe. Because there is an inductor, L4, between the inverter output and the grid, it's possible to vary the real power flow to/from the grid/load by varying the phase difference between the grid and the inverter output. Varying the voltage of the inverter affects the reactive power flow. You can get positive and negative real and imaginary power flow by varying the phase and amplitude of the inverter output. As far as I know, the inverter aims for zero imaginary power flow.

The firmware needs to carefully control the power flow so as to keep the bus voltage within reasonable bounds. It should always be above about 337 V, which is √2 x 230 V plus about 12 V for IGBT voltage drops and drop across L4. When the high V voltage Solar Charge Controller is pushing power into the bus, it gets pretty complicated.

2 hours ago, BritishRacingGreen said:

The transformer has a 1:8 ratio for this purpose .

The transformer ratio is 1:7 these days for 64 V models, which is the vast majority of the models now (sadly). In 58.4 V models, the ratio is still 1:8.

2 hours ago, BritishRacingGreen said:

Basic operation of grid-tie function :

...

1. Inverter samples the ac grid and locks its own output to voltage and phase of the grid.

2. Once locked , it operates a relay which ties the grid to the inverter output.

Yes. But after lock is achieved, the phase and amplitude of the inverter with respect to the grid is varied slightly, as noted above. It's good to lock phase before the relay closes, so it doesn't arc unnecessarily. It's not clear to me whether the inverter just goes for 230 V (or whatever output voltage is set), or it tries to match amplitude as well before closing the relay pair. (Active and Neutral are both switched together).

2 hours ago, BritishRacingGreen said:

3. Because of the very low impedance of the grid side , the inverter will now become a current source , rather than a voltage source.

I would say that the inverter is controlled in both phase and amplitude with feedback from both the grid voltage and load current. The very low impedance of the grid is blunted somewhat by the impedance of L4. L4 has reactance and little resistance, so that allows more comfortable control without wasting a lot of power. Otherwise, microvolt differences could cause tens of amps of current to flow.

2 hours ago, BritishRacingGreen said:

In the case of true gridtie , a feedback current transformer is monitored by the inverter , in order not to increase its voltage beyond the point of the injected current flowing into the main grid.

The Axperts are hampered slightly by not having a current sensor on the AC-in port. So the AC-in current has to be inferred/calculated.

 

6 hours ago, BritishRacingGreen said:

Hi @Coulomb, one of our members is currently having a problem with his synapse inverter. I quote his  problem and request  below :

Yes, Carl PM'd me about this too. I've been flat out today, I'll reply to his PM soon.

17 hours ago, Coulomb said:

don't believe it is, no. If it operated that way, the diodes would conduct only during the peaks of the grid sine wave, and that's terrible power factor. We can't get away with bad power factor over a few hundred watts these days, and less than that in Europe. Because there is an inductor, L4, between the inverter output and the grid, it's possible to vary the real power flow to/from the grid/load by varying the phase difference between the grid and the inverter output. Varying the voltage of the inverter affects the reactive power flow. You can get positive and negative real and imaginary power flow by varying the phase and amplitude of the inverter output. As far as I know, the inverter aims for zero imaginary power flow.

Thank you for this detailed explanation. Its quite complicated, but my laymans understanding here is that the dc-ac converter still operates, but the voltage and phase conditions are set to actually affect power flowing from grid into the machine for charging purposes. 

As my basic understanding grows, i develop new way of looking at internal inverter functionality, and new questions pop up. 

In fear of inundating @Coulombwith too many of these questions, i will limit to one at this stage.

Say we have 2kw capable pv power,a 5kw load, a full battery and good escom grid power (!) . 

I decide to switch to SUB mode, so to conserve battery power for tonights load shedding. 

So the 2kw dc  will blend  with grid to supply the 5kw.now how does the DSP prevents battery discharging? 

I see two ways:

1 the dsp shuts down the dc-dc converter, but that also shuts down possibility of  battery topup charge. Also creates problems when the grid falls away and we need a sudden discarge. Or is the DSP fast enough to cope with this adhoc. 

2 the ac blender pushes power onto the load, but as soon as battery discharge flow is detected, it throttles back a bit. 

Edited October 29, 20223 yr by BritishRacingGreen

1 hour ago, BritishRacingGreen said:

the dc-ac converter still operates, but the voltage and phase conditions are set to actually affect power flowing from grid into the machine for charging purposes.

Yes, exactly. It's easier to think of in DC terms, so pretend that the grid is a battery, and the inverter is able to produce a voltage a little lower than the grid-battery, so power flows into the inverter. The electrons just roll down hill, as it were, simplifying horribly. If the inverter creates a voltage just higher than the grid-battery, power flows out if the inverter.

In the AC case, there is phase as well as amplitude, and of course things are cycling 50 times per second. For some people, it helps to think of phasor diagrams, where there are vectors representing the instantaneous phase and amplitude of the inverter and grid, and these phasors are rotating at 3000 rpm. But you use a strobe light to freeze them, and you can see that there is a small gap at close to 90° between the two vectors, and that gap represents the voltage across the inductor. That voltage will be positive or negative depending on the relative phase of the two vectors, which the DSP has control over, as long as the grid is reasonably stable, and it usually is. Even in South Africa 😃. Now the voltage and current in the inductor are 90° apart, so that means that the current is roughly in phase with the two other vectors. So you get high power factor. Though if you change the amplitude of the inverter's vector, that pushes the inductor's voltage away from a right angle, so you can affect the power factor (importing or exporting VARs) by changing the amplitude. It's all quite tidy mathematically.

1 hour ago, BritishRacingGreen said:

Say we have 2kw capable pv power, a 5kw load, a full battery and good escom grid power (!) . 

I decide to switch to SUB mode, so to conserve battery power for tonight's load shedding. 

So this means that the PV is (hopefully) pushing 2 kW into the DC bus, and 3 kW is coming from the AC grid. The result is 5 kW, which exactly balances the load. Note that the 2 kW from the PV is getting to the load via the inverter, so there is power flowing in the DC→AC direction in the full bridge.

1 hour ago, BritishRacingGreen said:

now how does the DSP prevents battery discharging?

That's too easy; the DSP arranges the phase so that power flows from the DC bus towards the load. If the load increases or decreases by 1 kW, that's no problem, the grid instantaneously supplies the difference. That's Kirchoff's law, so it's instantaneous, and the DSP doesn't even have to make any changes, just keep that 2 kW chugging out to the load.

The only problem is if the load suddenly reduces by more than 3 kW. Say it reduces to 1 kW. Then the DSP has to realise that suddenly it has to reduce the PV power by half. The DSP has to do that, but its measurements aren't instantaneous, and control systems have lags (especially Voltronics' ones). So with only 1 kW of load and 2 kW pouring into and out of the DC bus, the inverter will instantaneously be pushing a kilowatt back into the grid.

2 hours ago, BritishRacingGreen said:

1 the dsp shuts down the dc-dc converter, but that also shuts down possibility of  battery topup charge.

I don't believe it will do this, though it might. If no battery charging is needed, it can open the buck converter. But generally the battery will always be on float charge, so the battery charge is regulated by adjusting the PWM of the buck transistor. As the bus voltage goes up and down with fluctuations in solar power, the buck converter adjusts in real time to keep the battery charge current close to where is needs to be (the so-called set point of the control algorithm).

2 hours ago, BritishRacingGreen said:

2 the ac blender pushes power onto the load, but as soon as battery discharge flow is detected, it throttles back a bit. 

As I said earlier, there is no active "blender" agent; it's just Kirchoff's law. The sum of all currents into a node have to add up to zero. This works at the instantaneous time scale, but also at longer time scales where we can think of an alternating current as being "constant" or "steady". The DSP is still busy, balancing the current out of the DC bus into the inverter with the current going into the DC bus from the PV boost converter. So there is a control system regulating the DC bus voltage as well.

Chapter 3 : The DC-DC MOSFET Drive Section

Knowing that the main PSU has caused (hopefully repairable) damage , its time to visit each subsystem on the Axpert Main board . The first one is the dc-dc converter mosfet  drivers . If you look at the power chain schematic in below , you will notice that on the battery side of the high frequency transformers there are 12 mosfets , four strings of three each.  The transformers has winding TX1 1-5-2 and TX2 1-5-2 . The pins 5 are centre tapped and they are connected to Battery positive (BAT+) . The transformer is switch via pins 1 and 2  which are connected to battery negative (BAT-) . 

 

 

The mosfets on TX1.1 (Q38/21/24)  and TX2.1 (Q20/11/17)  are driven by a phase_A signal . The mosfets on TX1.2 (Q19/26/40)  and TX2.2 (Q18/23/13)  are driven by a phase_B signal . These two phases are 180 degrees out of phase , in order to affect a push pull driver to produce ac for the transformer ,

Before we attempt to check the drive circuit , it is important to check whether any of the mosfets are blown or short circuit. To do so consult the Axpert MAX 7.2 service manual . Although we are not supplying any battery voltage onto the bus , it is important to check wether the mosfets requires replacement .

The schematic below is the mosfet and igbt driver of the dc-dc converter. Again I have consulted Maxo's schematic of the 5KW machine and checked it against the MAX 7.2 hardware , and again the circuit and component numbering are compatible , except for the mosfets and driver resistors. The 5KW has 4 mosfets per string while the max has only 3. So if you want to use this schematic for other Axpert models , note that the mosfets and mosfet resistors will be different.

 

You will note that an SG3525 chip is employed to generate the pahse A and phase B signals for the 4 mosfet strings. Noteworthy is the fact that the frequency and duty cycle of these signals are fixed and not under DSP control. The DSP can merely shut the outputs down in case of failure modes . This is done via U19 optocoupler , which pulls the shutdown pin (SD) to -12V.

-12V !!!!!!!!!!! oops, I realized that the driver section is not referenced to GND , but instead to -12V .  Very sloppy on my behalf , I only repaired the +12V rail  and the 15V rail , not the -12V and 5V rails. So back to the main PSU.  You will notice on the pSU schematic that regulation is accomplished only via the +12V rail . However , because the +12V is regulated , it infers that the transformer secondary windings for 15V , -12V and 5V are also regulated , but not as accurately as the +12V. For this reason , the -12V and 5V rails have been fitted with linear regulators (LM7805 and LM7912) . So I replaced the diode and electrolytic caps on the -12V rail ,  and tested the linear regaultors for shorts . Switched on , and I have a perfect -12V rail supply. To the mosfet driver section.

So because the driver supply is referenced to -12V , it means the total VCC of the SG3525 is actually 24V . you can verify that by reading voltage between pins 12 and 15 of the device.

Looking at the 12V and -12V supplies that feeds the SG3525 , I began to loose any hope that this device is still ok. I mean two rails had gone up 50 about 50V during the 'assassination'  of the PSU. So probably the SG3525 was hit by 100V , as opposed to a maximum supply of 35V that the device can tolerate. But the input amps from my bench supply did not suggest excessive currents being drawn. I measured the VREF of the device and it sat at 5.0V which is correct.

So I disabled the shutdown function by pulling the SD pin low via the opto-coupler , in the hope of generating mosfet driver signals.

Next I employed an oscilloscope to check whether I have any square wave signals on the A and B outputs (pins 11 and 14).  Sadly nothing . I touch the SG3525 device and I could feel it getting hot . No wonder , its faulty. But not fried. So I  will have to buy a couple of chips next week (R10.00 @ Mantech)  and replace the faulty device. The other solid state devices on the driver circuit appears to be fine.

So sadly that's it for this board until mid next week.

But nothing prevents me to use a good board to check the type of driver signals I will expect. So I fired up another board and the signals are as shown below :

Just what we will expect : the two signals are 180% out of phase , and you will also note a dead time between transitions. This dead time time is essential to allow mosfets to be switched off when other are switched on. The duty cycle is close to 50% , the  frequency has a time period of about 24uS.

 

As far as voltage is concerned :   must be noted that the driver is reverenced to the system supply rails (+12V and -12V) , but the mosfet gates must be referenced to their source pins which are connected to battery minus (BAT-) . This is of course the reason why the drive signals are pulsed thru isolation transformers TX5 and TX8.   The secondary windings of each phase transformer (TX8 and TX5)  tied to BAT- as shown in the schematic.

The image below depicts the voltage of a driver signal at the mosfet gate pin. The values are of course referemced to BAT- . You will notice the positive magnitude is about 14 volts (mosfets turns on) , while negative  it is - 12 volts (mosfet turns off) .

 

These signals are super important and this is the only way to verify that when you are going to replace expensive mosfets , that you have left little to the imagination.  

Of course one needs to check these signals at each gate of the twelve mosfets . It is recommended to use 2 channel oscilloscope in order to verify that each string is in correct phase as well. One channel on phase A  and the other channel on the mosfet gate . If in phase then the mosfet is driven by phase A, if out of phase it is driven by phase B.

 

EDIT : the terminology phase_A and phase_B is not official , it has been created by me to reference driver signals thru out the schematic.

 

 

Edited October 29, 20223 yr by BritishRacingGreen

The 'assassinated' main PSU temporary repair now looks even more scarier after I repaired the -12V . This mess will only be addressed when the repair has met critical mass , i.e. if the board will become serviceable.

It reminds me of my engineering mentor back in the day . He insisted you must not over design or over repair something . If you design an F1 car , it must be able to win a race , but just after passing the chequered flag , the car must overheat, the gearbox must disintegrate , the suspension must fall apart , and the wheels must fall off with only a thin layer of rubber left on tires. Otherwise its over-designed and over-engineered !

8 hours ago, BritishRacingGreen said:

Again I have consulted Maxo's schematic of the 5KW machine and checked it against the MAX 7.2 hardware , and again the circuit and component numbering are compatible , except for the mosfets and driver resistors.

Actually, your schematic trace reveals some serious problems with this part of Maxo's schematic. For example, the diodes and resistors in series with the collectors are swapped top to bottom. I always thought it strange that there are two 1μF capacitors in series; the middle of these should go to CH- (BAT- in your model without the reverse battery MOSFET protection). I'll be busy fixing these soon. Thanks for the prompt.

Nearly a week has passed since I dotted down any progress. So I have finally got the SG3525 chip and replaced it , my entire battery side mosfet and  battery side igbt drive sections is up and running and each and every device has been verified at its gate signal for correct drive waveform. 

PCB rework on this main board is a pain . I had to actually cut out the faulty  16 pin SG3525 and removed each pin individually .  I also decided to add an ic socket , a high quality one , time will tell whether environmental conditions e.g. heat will be ok for this , but I am confident.  I am using solder wick for repair , wish i had de-soldering station like Coulomb and Weber  , but too expensive here by us. Unfortunately you get wick and then you get wick. Some are bad at sucking , some better. I have found that thermal transfer from the soldering iron onto the wick is super  important , and i have discovered that adding a small bit of solder to the tip and wick goes a long way . obviously not too much , otherwise you saturate the wick. So sometimes you need to add a bit of solder to remove a bit of solder !!!!!!  Also the conformal coating is nasty , so I get that out of the way with surgical spirits and maybe bit of acetone at times. 

There is one thing that is bothering me though . When I enable the mosfet/igbt battery side drives , the power consumption is like 20 watts !! for drive signals !????? The -12V linear regulator gets hot , and various components on the main board get warm as well. But here's the thing , on my other reference board I get exactly the same thing also about 20 watts added. So I decided to step down from this for the time being , but will probably revisit this again for the good order.

So that's it , I have so far repaired the main PSU and the battery side drive section. Feeling confident slowly that I will bring up this machine.

I decided to redraw the power chain chain schematic on kicad as well. The important  thing at the moment is that the schematic depicts the relationhip between gate signals and source reference . Its important to identify thi , because obviously one of the functions of the driver subsystem is that they are isolated and referenced to the source (mosfet) or emitter (igbt) .

 

So next up is the AC side IGBT drive section , and I know there are problems , because the very first first thing I did on this board was removing the drive diode to the igbt drive transformer, cause it was getting hot.  Let us see what can of worms opens up here in my next chapter.

Chapter 3 : The AC IGBT Drive Section

 

Haven't even started on a schematic trace here , hope to add it during next week.  But for me , the show already stops at the very beginning . The 12 SMPS flyback transformer provides an ac signal  which is diode half wave rectified , which in turn drives the primary of TX7 , the IGBT drive transformer. So I thought i will put the diode back because my rail supply is back to 12V again, in the hope that the igbt transformer was just incapable of coping with the faulty high voltage. Solder D20 back on , switched on and smoke came from the transformer . Ai, Ai , Ai. Brand new difficulties and possible cul-de-sac lying ahead. 

So in retrospect , when the SMPS failed and decided to throw max voltage on the bus , this TX7 transformer actually failed first and clamped the rail. Thats the reason why initially when i removed the hot diode D20 , all hell broke loose as i described in chapter one !!!!

I hate transformers , probably because I know too little about them.  The worst news is this transformer is Voltronics proprietary , so no ways I can even think about getting a spare . Only alternative is reverse engineering the device , and re-building from scratch. If you see what Coulomb and Weber do on their AUS forum , its clear that these guys fathers have raised them not to be scared. So inspired by them and others , I decided to rebuild this show-stopping sun-of-a-gun , how difficult can it be?.

Very fortunately , a member by the name of Holmes on Coulombs AUS site has done something similar , so I had a good idea what was lying ahead.

This time I did not have the luxury to cut the devices legs to desolder, so my solder wick skills was tested again , this time I managed well , but it takes a long time , and you need to be patient.

I fired up my gas-braai and cooked the transformer in boiling water for about 30 minutes. That softened up the glue sections , so I was able to dissasemble the split core halves. Next I removed the Mylar tape layer and started to unwound the the coils , obviously documenting it carefully . I am going to make proper notes of my rough sketches and publish it here at a later stage.  

Rewinding these devices is not rocket science , but one must be careful to keep the windings flat , or your next layer will become unmanageable. There is one primary winding , 21 turns of .3mm enamel copper wire , then there is three secondary windings each 35 turns , each winding section is nicely insulated with about two layers of Mylar tape. oh , and of course testing for isolation between windings as well (insulation).

I was not confident enough to just plonk it back onto the mother board . So I bench tested it , in order to both check  voltage gain in terms of turns ratios as well as polarity of winding in relation to each other . It passed all test surprisingly with flying colours .

So I soldered it onto the board , switched on , and I got the secondary voltages that the drive section desires . Next post I will post the drive schematics and show the waveforms of the secondary winding in more detail.

 

EDIT : oh , I forgot , all did not go plain sailing , when I first bench tested the re-winded transformer , the core got hot !. Long story short , there was a nasty air-gap formed by me not putting the split core properly together. So this is a forward mode transformer , it does not store and release energy like a flyback transformer (which is really a multi winding inductor) .  So the air gap caused inductance to lower , huge magnetizing current flow in primary.  So I made sure to mechanically secure the split halves by a healthy layer of Mylar tape.

 

Edited November 5, 20223 yr by BritishRacingGreen

12 hours ago, BritishRacingGreen said:

I am using solder wick for repair , wish i had de-soldering station like Coulomb and Weber  , but too expensive here by us.

I actually go to Weber's lab and use his desolderer 🙂

His is the best of the Chinese cheapeys, as described in this and subsequent posts:

http://forums.aeva.asn.au/viewtopic.php?p=65255#p65255

IF you decide to take up repairing more seriously, you may find the relatively modest investment (relative to the really good ones) worth while.

Quote

So sometimes you need to add a bit of solder to remove a bit of solder !!!!!! 

Yes! I have a really thick tip (diameter around 5mm) for heavy work. When I can't be bothered schlepping over to Weber's lab, I've been known to use solder wick, and even paper clips to clear the holes of solder:

From: https://www.diyelectriccar.com/threads/tcch-elcon-charger-troubleshooting-and-repair.90162/post-743089

That's from my electric vehicle charger repair and reflashing interest.

Edited November 6, 20223 yr by Coulomb

In some cases with some boards with heavy copper fills, I have previously resorted to using a soldering iron from both sides, before trading one for a compressed air gun to blow out the solder. It works really well on stubborn vias, but be prepared for a cleanup.

Having proper equipment is really worth the investment, though!

Edited November 6, 20223 yr by P1000

1 hour ago, P1000 said:

In some cases with some boards with heavy copper fills, I have previously resorted to using a soldering iron from both sides, before trading one for a compressed air gun to blow out the solder. It works really well on stubborn vias, but be prepared for a cleanup.

Having proper equipment is really worth the investment, though!

Thanks, i have notice some areas which is 4x4 territory.  What wattage solder iron you reckon is a good fit.? I want to complement my existing 60w one with somethin beafier.