Hey all,
I am building a dual H Bridge using some MOSFETs for a project. It should be controllable by the 5V of an Arduino and should control some heavy motors.
I came with the following circuit but I'm doubting whether the 20V max gates of the NFETs will be exposed to any inductive spikes.
Could someone see whether the flyback configuration is enough or should I use another H Bridge circuit.
Op's image:
Your upper MOSFETs are upside down. Look at the body-diode inside the symbol to see which way to put them.
Incidentally, the body diodes act as flyback diodes so you usually don't need to add any diodes to an H-bridge.
The lower half of the circuit is very unusual. I've never seen that before. If Q3 is on, it holds its drain at almost zero volts. That's connected directly to the gate of Q4, holding it off. Good, but now how do you turn Q4 on? There's nothing to pull down the gate of Q3.
MorganS:
Good, but now how do you turn Q4 on?
The gate of Q4 is pulled high when the gate of Q1 is pulled low (it's drain becomes high).
100k resistors makes that circuit sloooow (not good for PWM).
Better change them to 1k.
Simple circuits like this don't have shoot-through protection.
Better wear protective glasses when you power it up.
Leo..
For a MOSFET bridge or half-bridge freewheel diodes are never needed (although sometimes schottky
diodes are added for increased efficiency), as power MOSFETs contain a diode as an integral part of
their structure.
This is not true for IGBTs or BJTs, where diodes need to be added (some devices include
them as part of a multi-die package, particularly IGBTs).
If you have a single MOSFET its body diode is in the wrong place to act as a freewheel, note.
I just simulated the circuit in LTSpice. It appears to work with a purely resistive load. But add some inductance to simulate the motor and it starts to look bad. Applying a simulated PWM waveform to one side of the bridge with the other side off will give some shoot-through when switching on and when it switches off the energy stored in the motor inductance bounces backwards and forwards flipping Q3 and Q4 on and off very rapidly.
MorganS:
Op's image:Your upper MOSFETs are upside down. Look at the body-diode inside the symbol to see which way to put them.
Incidentally, the body diodes act as flyback diodes so you usually don't need to add any diodes to an H-bridge.
The lower half of the circuit is very unusual. I've never seen that before. If Q3 is on, it holds its drain at almost zero volts. That's connected directly to the gate of Q4, holding it off. Good, but now how do you turn Q4 on? There's nothing to pull down the gate of Q3.
The more I look at that circuit the more I suspect it will just pop half the MOSFETs when you first use it.
There's nothing to bias the bottom switches off hard enough - 100k will not win against a motor
winding resistance!
A can see a failure mode where both bottom switches come on together and massive currents flow on the side with an active high-side switch...
