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Flyback MOSFET clamping diodes

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kathmandu

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Hello,

I wonder if I could use a simple diode clamping circuit instead of the classical RCD snubber for a flyback MOSFET switch. Both RCD and bare diode are dissipative circuits hence I don't mind using a higher current rated diode as long as the MOSFET drain voltage is clamped (for sure) at Vcc level. The RCD snubbers still allow some transient voltages across them and I want to avoid that (I want to use a lower voltage rated MOSFET).

Here are the classical (RCD snubbers) and the suggested (diode only) solutions:

flyback-snub.png

Btw, the lower clamping diode is the MOSFET built-in (body) one. Thanks in advance for any oppinion.

- - - Updated - - -

I forgot to mention: I'm talking about a low power (5W) 48VDC to 12VDC flyback converter.
 

Clamping drain voltage "at Vcc level" means you are shorting flyback output voltage. Simply wrong. By the way, it would neither work in forward converter because it increases transformer reset time unwantedly

The MOSFET body diode will never conduct in normal operation and has no clamping effect. If you look harp, you'll notice that its polarity is opposite o clamping diode.
 
Looks like the high-side snubber is doing a similar task. When the MOSFET is turned off, the snubber capacitor is discharged hence the snubber diode is actually shorting the primary winding. Anyway, I thought that a flywheel diode should make a short-circuit on the primary side. I surely have some misconceptions which need to be addressed.. ;)

That's right, the MOSFET body diode is not conducting during normal operation (I was under the influence of a half-bridge operation).
 

Looks like the high-side snubber is doing a similar task. When the MOSFET is turned off, the snubber capacitor is discharged hence the snubber diode is actually shorting the primary winding.
But not for too long. Just for the initial condition, which is nothing.
Anyway, I thought that a flywheel diode should make a short-circuit on the primary side. I surely have some misconceptions which need to be addressed..
If short circuit is on the primary, what is the voltage at the output ?
 

If short circuit is on the primary, what is the voltage at the output ?

That "short-circuit" it's just a discharging path for the primary current. As you know, the primary is already "loaded" (at the rated input current) when the MOSFET is turned off. Despite that short-circuit (due to the clamping diode), the current still flows through primary (as you can't stop it abruptly) and will eventually fade away.

However, the energy is stored as magnetic field in the transformer's core air gap during charging stage (when the MOSFET is on).

Maybe you have to design the rest of the circuit (transformer, duty cycle) accordingly if you want to use such a clamping diode (as the discharging stage is shorter), but I think it's doable. At least in theory..
 

That "short-circuit" it's just a discharging path for the primary current. As you know, the primary is already "loaded" (at the rated input current) when the MOSFET is turned off. Despite that short-circuit (due to the clamping diode), the current still flows through primary (as you can't stop it abruptly) and will eventually fade away.
If the energy is being built up in the primary side and discharged (very very very slowly) also in the primary side, how does the secondary side get energy ?

Your transformer will end up saturated in a few cycles after is powered up.
 
Last edited:

If you don't want to use an RCD clamp, the other common method is to use a zener clamp. This requires two diodes across the primary, one is a zener, which sets the transformer's reset voltage, and the other is a standard high voltage switching diode that is able to handle the current flow during the reset period.

Example:
Simple Flyback.png
 

@GeorgesWelding:

Many thanks for posting that schematics; I've seen that clamping topology before but still don't know how effective it might be (the clamping voltage level it's still high for my expectations). In the circuit above, the zenner (TVS) is rated at 180V while the input voltage is 110VAC (rectified).

I'll try to find more informations on this subject as I didn't receive any reasonable explanation yet.
 

I'll try to find more informations on this subject as I didn't receive any reasonable explanation yet.
I thought you know how the flyback topology works, but it seems that you do not, so I will give you a short explanation. By the way, the reason you do not understand the flyback is because you do not understand the working principle of the transformer, so probably is better to search for the transfromer first.

When the FET is ON, energy is beeing built up in the transformer and hence, stored in it.
When the FET is OFF, you want to release that energy in the secondary side charging the output capacitor.
The snubber circuit is there just for the sake of the leakage inductance, to provide it an alternative path to relase its energy in.

You do not want to short the primary because the energy in the transformer would not decrease, but stay constant. Then, the next cycle you add more energy to the transformer and so on, until it get saturated in a few cycles.
 

Are you sure about the above explanation?

The snubber circuit is there just for the sake of the leakage inductance, to provide it an alternative path to relase its energy in.
If it's just like that, why bother using "an alternative path" of a single (flywheel) diode right across that inductance?

You do not want to short the primary because the energy in the transformer would not decrease, but stay constant.
So what you're actually saying is that if I'll keep the short-circuit on the primary side indefinitely, the energy stored in the transformer would stay constant (and would not decrease) forever?


Anyway, what am I actually shorting with that flywheel diode (or the RCD snubber, for that matter)? On your first sentence, the short-circuit is supposed to be applied to leakage inductance but on the second one you're talking about the primary inductance.

When the FET is ON, energy is beeing built up in the transformer and hence, stored in it.
If the energy it's stored in the transformer (and that's what I have told you few posts above), why does the primary/leakage inductance short-circuit really matter?
 

I thought you know how the flyback topology works, but it seems that you do not, so I will give you a short explanation.
Btw, I was not asking for the flyback principle but the reason why a simple diode can't be used as a clamping device.
 

If it's just like that, why bother using "an alternative path" of a single (flywheel) diode right across that inductance?
What? Not a single diode. A capacitor and resistor (to form a resonant circuit) or a zener diode, both having a simple diode to redirect the current.

So what you're actually saying is that if I'll keep the short-circuit on the primary side indefinitely, the energy stored in the transformer would stay constant (and would not decrease) forever?
Yes, as long as the short circuit is applied.

Anyway, what am I actually shorting with that flywheel diode (or the RCD snubber, for that matter)? On your first sentence, the short-circuit is supposed to be applied to leakage inductance but on the second one you're talking about the primary inductance.
Leakage inductance is usually small, that is why I made a basic assumption neglecting it, and hence, the short would be applied directly in the transformer. But then, you would have said, why snubber circuit anyway ? Then I gave you the reason in advance.

If the energy it's stored in the transformer (and that's what I have told you few posts above), why does the primary/leakage inductance short-circuit really matter?
Including the leakage inductance in the calculation and using your "flywheel diode"(short circuit), the current in that leakage inductance would increase further due to the reverse voltage from the output.
 

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