I have a DROK adjustable power supply 110-230VAC to 0-48VDC which now outputs only 0-5V. No visible damage inside so I thought I'd understand the circuit before proceeding. Some of the components don't have markings or are not visible. T2 might be a pulse transformer. There may well be errors in my schematic.



First question: the main supply seems like a straightforward half bridge converter, except the output inductor L1 (big toroid) is on the low side of the dc output. All the examples of this topology I've seen (not that many) have it on the high side after the diodes. Why might they have put it on the low side?

There's an auxiliary supply involving T3 and a DK912 chip, which according to the ~english datasheet I found is a primary side flyback type AC-DC Switch Mode Power Controlling IC.


The way this is connected doesn't resemble the datasheet sample application:

In the drok circuit it seems to be generating two independent low voltage supplies, one for the fan and one for the main TL494 switching controller. It would be great to know vaguely how this subcircuit works, what T3 is, and where the DK912 gets feedback to set its output level.

 

Hi,
everything is OK with big inductor. It will work same way on both poles.
You was right, thre's two independent power supplies, first one is flycack based on the DK912. First of all check voltages on C23 and C22, should read about 12V on both capacitors.
Second power supply is based on the TL494, there's a lot of application notes on the net. Check power Mosfets STW20 , check voltage on the C5, C6 , should read about 150V

 

Hi,
everything is OK with big inductor. It will work same way on both poles.
You was right, thre's two independent power supplies, first one is flycack based on the DK912. First of all check voltages on C23 and C22, should read about 12V on both capacitors.
Second power supply is based on the TL494, there's a lot of application notes on the net. Check power Mosfets STW20 , check voltage on the C5, C6 , should read about 150V

Thanks for the help! C23 and C22 are both at ~14v. TL494 is getting vcc=14v, and it's producing its reference output of 5v, so it is somewhat alive. The mosfets check out ok. C5 and C6 are 165V. Any ideas what to look at next?

 

OK, and a question: is the zero to five volts REGULATED?? and is it adjustable with the same adjuster pot? Can it deliver the same current?? It could be that the DC supply to the regulator has failed, if the output is stil adjustable and regulated.

 

OK, and a question: is the zero to five volts REGULATED?? and is it adjustable with the same adjuster pot? Can it deliver the same current?? It could be that the DC supply to the regulator has failed, if the output is stil adjustable and regulated.

From the datasheet:

The TL494 internal 5-V reference regulator output is the REF pin. In addition to providing a stable reference,
it acts as a preregulator and establishes a stable supply from which the output-control logic, pulse-steering
flip-flop, oscillator, dead-time control comparator, and PWM comparator are powered. The regulator employs a
band-gap circuit as its primary reference to maintain thermal stability of less than 100-mV variation over the
operating free-air temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal
reference and preregulator; 10 mA of load current is available for additional bias circuits. The reference is
internally programmed to an initial accuracy of ±5% and maintains a stability of less than 25-mV variation over
an input voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the
input and tracks it.

In this circuit, the 5V is present and stable so the regulator is working. The 5V gets used in the setting of the control signals that determine pulse width, and it also goes across the output adjustment pot -- the 0-5V signal that comes back is involved in setting the FB pin to control the output.

 

imho, DK912 and TL494 are OK. Check all electrolytic capacitors, an ESR checker is usefull to check incircuit. By the way ,do the voltage setting pot is wired same way as it was before the fault?

 

imho, DK912 and TL494 are OK. Check all electrolytic capacitors, an ESR checker is usefull to check incircuit. By the way ,do the voltage setting pot is wired same way as it was before the fault?

Finally got some time to work on this thing. I checked all the electrolytics on the output side, except for a couple of the 1000uFs that had a mountain of solder that my iron had trouble with. Also checked the common cathode output diode. All good. I did some probing of the tl474 - in particular the error amplifier inputs and the feedback pin. The voltages are shown on this updated schematic.



1IN+ is at 0.464v, so it's getting the correct scaled down output voltage.

1IN- is at 2.046v which is the correct level from the 5V adjustment pot.

2IN+ is grounded but it's at 0.005v relative to the main output ground rail to which it is connected by a 1mm trace about 5 inches long, maybe 0.05ohms, meaning there's 100mA flowing there, which seems odd.

2IN- is at 0.024v which is 5v REF divided down correctly to ~zero volts at the shunt.

FB is at 0.077 which should give maximum duty cycle so it would seem the TL494 is either not switching or there's something wrong with the mosfet driver or downstream.

 

Check resistors R6, R8 . And T2 secondary resistance , 1-2 and 3-4.

Do you have an oscilloscope? Check voltage on the T2 primary. Should see a square wave with small dead time.
Also check voltage on the 494' E1 and E2. Should see both voltages of same shape and value

 

Check resistors R6, R8 . And T2 secondary resistance , 1-2 and 3-4.

Do you have an oscilloscope? Check voltage on the T2 primary. Should see a square wave with small dead time.
Also check voltage on the 494' E1 and E2. Should see both voltages of same shape and value

R6 was failed open, so I replaced both it and R8. The output is still limited to 5V, but the adjustment pot works (0-5V).
T2 secondaries are both 0.1 ohms.

T2 primary:


TL494 CT:


TL494 E1(purple) E2(yellow):

 

It seems, everything is OK with the T2 primary.
Check resistors R5, R6 and diodes D7, D8

 

It seems, everything is OK with the T2 primary.
Check resistors R5, R6 and diodes D7, D8

Yeah, I checked those before, they're good.

 

Check the waveform with osciloscope on the T2 secondary 1-2 and 3-4.
Keep in mind, those are at mains side

 

Check the waveform with osciloscope on the T2 secondary 1-2 and 3-4.
Keep in mind, those are at mains side

T2 secondary 1-2:



Pins 3-4 is the same. Looks pretty much the same as the primary, just ~30V pp instead of 25.

I tested some more parts downstream from T2 and found that the big red film capacitor C15 is bad. Should be 4.7uF and is testing at 100pF.

Can you describe the function this performs in the circuit?

Any suggestions for where to get a replacement? Digikey has no exact match and only 2 items with similar specs but they have 4 pins and are much larger and won't physically fit on the board. Then there are sketchy options from ebay or aliexpress.

 

C15 lost the capacity. That can explain low voltage at the output.
The capacitor limits current pulse duration thru open power transistor, imho.
You can search the capacitor for lover voltage- 300V or even 250V. It's should be metalyzed polypropylene type. Look for these types : MKP, CBB13, CBB22, MPA, MPT, CBB20, MPX. Two capacitors 2.2 uF each in parallel also can be used. Necessarily made of polypropylene

 

I replaced C15 and the power supply is now working properly again. Many thanks for your help.

 

I have a similar power supply (0-36V 480W) and i was about to draw the schematic until i found your post.
What software did you use to draw both schematics (free / online)?
As i want to make some modifications to my power supply, are you willing to share both schematic CAD files so i can check/modify them against my power supply?

 

Here is the kicad schematic file. I'd be interested in hearing about your modifications.

 

I am not able to open it. Could you attach it as a PDF?

 

Here is the kicad schematic file. I'd be interested in hearing about your modifications.

There appears to be a component missing on the bottom right??

 

Here's the symbols for the TL494 and the gate drive transformer.