H-bridge blowing up in my face...

Alec_t

Joined Sep 17, 2013
12,309
when the lower MOSFETS are off, it doesn't matter what the gate bias is on the upper MOSFETs.
True if both NFETs are off, but I don't like the idea of having a PFET gate floating when one NFET is on. Noise/glitches could cause spurious conduction of the 'off' PFET, hence shoot-through.

Edit:
If floating PFET gates can be tolerated and the NFETS always have an active drive then the circuit could be reduced to its bare bones. Simulation shows the switch-off spikes being clamped close to the rails by the body diodes.
SolenoidBridgeMinimal.jpg
 
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Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
True if both NFETs are off, but I don't like the idea of having a PFET gate floating when one NFET is on. Noise/glitches could cause spurious conduction of the 'off' PFET, hence shoot-through.

Edit:
If floating PFET gates can be tolerated and the NFETS always have an active drive then the circuit could be reduced to its bare bones. Simulation shows the switch-off spikes being clamped close to the rails by the body diodes.
View attachment 248904
Nice! ... I guess this circuit is fine for a solenoid working very at low cycle frequency... but not for a brushed electric motor! The commutation spikes and inductive kickback could indeed cause trouble, couldn't they?
 

Alec_t

Joined Sep 17, 2013
12,309
The spikes would certainly be more numerous with a brushed motor, but in principle they would be clamped by the body diodes in the same way as for the solenoid.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
The spikes would certainly be more numerous with a brushed motor, but in principle they would be clamped by the body diodes in the same way as for the solenoid.
I understand, but my point is that the pFets could be sporadically triggered due to the much higher frequency of the spikes caused by the brushed motor.
 

BobaMosfet

Joined Jul 1, 2009
1,896
So I built this h-bridge driver for a solenoid valve, and it worked beautifully for a while.

After my third circuit, suddenly things went boom and magic smoke came out of the p-fets (and sometimes of the n-fets too)... I don't know what changed. Maybe I've been lucky and the circuit's been working on the transistors' tolerance thresholds until now.

My take is that for a brief moment M1 and M4 (or M2 and M3) are short-circuited and that's why things are burning up ... I can find no other reason.



CW and CCW are 0 to 5V signals, btw. Zeners are rated at 18V.

What am I doing wrong? Is there a way to add simple parts to it so as to somehow de-phase M4 and M1 (and M3 and M2) when they're triggered on/off so that they'll never short circuit?

I was thinking of placing a diode and a cap and resistor in series in inverse parallel at each transistor's gate to delay their turn-on time whilst keeping their turn off time almost unaffected.

Something like this:


Is this a good idea? Anyone have a better one, without requiring that each transistor be switched individually?
Just curious- why are you using solenoids (inductor, coil driven)? No choice in the matter?
 

Orson_Cart

Joined Jan 1, 2020
76
Generally speaking ( as a power electronics engineer ) self commutation of mosfets per the above is a bad idea - especially for varying loads and voltages - there is a reason half bridge driver chips exist - they provide guaranteed dead time and surety of gate drive levels - also you can buffer them to ensure the gate is held low when the other device turns on.
You can design your own gate drive ckts - as we do - but we find that 20+ years of design experience means that ours work under all expected conditions ...
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
Generally speaking ( as a power electronics engineer ) self commutation of mosfets per the above is a bad idea - especially for varying loads and voltages - there is a reason half bridge driver chips exist - they provide guaranteed dead time and surety of gate drive levels - also you can buffer them to ensure the gate is held low when the other device turns on.
You can design your own gate drive ckts - as we do - but we find that 20+ years of design experience means that ours work under all expected conditions ...
Thanks ... I learned that the hard way ... And although the simplest circuit shown in post #61 is indeed practical, it doesn't offer control over deadtime. That's why I'll be playing it safe and use the circuit shown in post #26, sans the redundant R7 and R8.

That circuit will be controlled by an MCU, so I'll be able to program as much deadtime (I prefer the term "leisure time" better :) ) as necessary to make things safe.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
I have a new challenge for this circuit....

What would be the simplest way of protecting it against a short circuit condition? That is, assuming that the H bridge itself works fine and it's guaranteed that two mosfets on the same side of the bridge never turn on simultaneously, what is the best way to make sure that the circuit is never overloaded when the solenoid itself is somehow short circuited?

The circuit works with two parallel sets of 10 AA batteries connected in series that together deliver 16VDC. The arrangement doesn't sound like much, but it's powerful enough to cause a small fire if shorted ... I know that from personal experience.

I'd like to limit the current output to 4 Amps, if possible.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
How about a fuse, if we are talking about a possible dead short and not a minor overload?
Yes, of course a fuse is an obvious answer. But I'd like to use a current limiter instead because the circuit that I'm building is going to be placed in a sealed waterproof enclosure that will not be easy to open and reach the components inside.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,513
Never mind ... I've just figured that they're there to limit the amount of current going into Q1. And placing them anywhere else doesn't work.
 

kubeek

Joined Sep 20, 2005
5,768
Yes, of course a fuse is an obvious answer. But I'd like to use a current limiter instead because the circuit that I'm building is going to be placed in a sealed waterproof enclosure that will not be easy to open and reach the components inside.
The problem is that in worst case the limiter needs to dissipate half the total available power, so 16x4/2=32W and that is not easy in a sealed enclosure, unless you add an overtemperature shutdown or time limit.

Another way would be a foldback limiter, which once above the threshold limits to much lower current and resets when the current decreases even further, i.e. when the load is disconnected.
 
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