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Three phase bridge driver circuit

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electronicsman

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This is a small project to control a motor using 3 phase bridge circuit. My only question is let us say i have to switch one top leg and one lower leg based on hall sensor, let us say it is H1L3. This is the configuration i tried H1 is controlled based on PWM and the lower leg is configured as output and made permanently low. Let us say i give PWM with 100% duty then the complete voltage is applied to the motor. Am i correct on this? Suppose if i control the H leg with 50% duty then half the voltage will be applied. If my above two assumptions are correct. Then I have one more scenario let us say i control the H leg and L leg both with PWM then how the behavior will be? I mean to say H leg with 50% duty and the Low leg at 50% duty. What will be the voltage applied to the motor? Is it simple calculation or it has to be derived using some equation? Please advise.
 

...This is the configuration i tried H1 is controlled based on PWM and the lower leg is configured as output and made permanently low. ....

The bridge circuit allows you to use a positive supply but current in the load can be either positive or negative (current in the load can change direction). You need to consider the following:

1. No H1L1 (or H2L2 or H3L3) are turned on at the same time; that will short the power supply and no current will flow through the load.

2. You need to set up dead time; a small gap is given when all the pulses are off at the same time.

3. You need one signal for each pair; H1L1 (or H2L2 or H3L3) are driven by individual signals that have complementary outputs (with dead time): when H1 is on L1 must be off. During dead time both will be off but both will never be on at the same time.

4. You need to use PWM for sine wave approximation. Else you can use 90% PWM to drive all the phases with square waves.
 
Hi,

some issues:
Let us say i give PWM with 100% duty
Many high side driver circuits can´t go to 100% because of the bootstrap circuit. Use 98% max. duty cycle

then the complete voltage is applied to the motor.
Theoretically yes. But ther is the above problem. And a motor won´t withstand this because it needs to be driven with AC. It will be killed very soon.
But if you have a resistive test load, then yes.

Suppose if i control the H leg with 50% duty then half the voltage will be applied.
With a resistive test load: yes. But not with a motor, because it is inductive.
It will generate it´s own voltage. If the lower MOSFET stays ON (slow decay mode), then there will be a small negative voltage across the inductance, as long as there is magnetic energy in the coil.
In reality the RMS voltage will be around 50% of the supply voltage.

H leg with 50% duty and the Low leg at 50% duty. What will be the voltage applied to the motor?
(fast decay mode). When both MOSFETs are OFF, then there will be about the negative sully voltage across the motor winding as long as there is magnetic energy in the coil.
--> The resulting averaged winding voltage will be close to zero. Low current, low torque.

These are raw estimatins.
It dependson
* motor: circuit, inductance, (DC) wire resistance, RPM, mechanical load..
* driving ciruit....

Klaus
 
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