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I'm having trouble with a flyback converter I got. My output voltages are not what they are supposed to be. Flyback schematics is below:

enter image description here

If you cannot see very clearly, primary number of turns is 200 and 15 and 5 on 15V and 5V secondaries respectively. Problem with my output voltages is that they are not what they're supposed to be. 15V is too hight and 5V is too low.

My voltage waveforms are:

CH1: Command signal

CH2: VDD (supply to chip)

CH3: 15V output

CH4: Primary voltage

enter image description here

And also with 5V output on CH4:

enter image description here

Anyone has any idea what is happening and what might be the problem?

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  • \$\begingroup\$ "5V is too low" what erm like 5 billion volts too low or 1 nano volt too low? Or maybe somewhere in between? \$\endgroup\$ – Andy aka Jan 19 '17 at 16:19
  • \$\begingroup\$ It's 3.5V-4V... \$\endgroup\$ – MarkoP Jan 19 '17 at 16:21
  • \$\begingroup\$ Ditto what HatimB said. Also use bigger caps (like 100uF or higher) for better regulation on 15V side. One more thing: Use a dummy load on 5V output (like a 1k resistor). \$\endgroup\$ – Rohat Kılıç Jan 19 '17 at 16:52
  • \$\begingroup\$ What is the load on each rail? What is your leakage inductance? \$\endgroup\$ – winny Jan 19 '17 at 16:54
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    \$\begingroup\$ What does your FB pin read? \$\endgroup\$ – winny Jan 19 '17 at 16:58
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You feedback signal is the 5V_SMPS, but I don't see anywhere in your schematic where SMPS_GND is connected to POWER_GND.

The transformer could be a contributor to this error since it is not single sided (3.5V instead of 5V and 20V instead of 15V).

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According to the schematic, there are 5 turns for the 5V output, and (5 + 15) = 20 turns for the 15V output. The ratio 5:20 is ways off. It should be 5:15total (or perhaps 5:14 when accounting for the diode drops).

Now couple that with the D1 18V Zener clamping on the +15V, that may explain the high voltage on the +15V and low voltage on the +5V.

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The problem is solved. It was the resistor RS5 that provoked a voltage drop on 5V winding.

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