giovaniluigi
Member level 1
I'm doing a circuit to control a BLDC motor using MOSFETs.
The Motor should not drive more than 10A @ 24V and the mosfets are the FQP50N06L.
The motor has HALL sensors for commutation control so BEMF is not monitored.
The motor speed is controlled using a 20KHz PWM on bottom FETs.
I designed the driver of each half-bridge using an IR2101.
For the High Side of the bridge the IR2101 uses a diode and a capacitor to do the trick to switch the MOSFET gate.
The only problem is that I'm with problems to calculate and understand exactly this bootstrap capacitor.
Two years ago I designed a BLDC controller using an IC similar to the IR2101. I used the IR2110 which is "stronger" but not needed in this case. Without knowing the capacitor valule I started to add one electrolytic capacitor of 1uF. The motor didn't worked. Then I replaced by a 10uF capacitor. The motor started to spin but very weak. Then I replaced by a 47uF capacitor. The motor worked but when I added some load the commutation were "skiping" then I replaced again the capacitors by a 100uF. This way the motor works always and with a great torque.
Two problems:
1 -
The MOSFET has a low gate charge requirement. In my opinion 100uF for this is an insane value.
According to my calculations the minimum value would be about 300nF.
I'm using a common electrolytic capacitor. The ESR of the capacitor maybe the problem ?
2 -
Motor doesn't start. I need to manually start it, make it spin a little (half a turn) to it start spinning.
That seems to be happening because the bootstrap capacitor too. No enough charge on it.
Is there any special procedure to start a BLDC motor driven by this simple bootstrap strategy ?
I guess not because I saw a lot of schematics on the internet with nothing special...
Some used an additional power supply of -15V to supply the bootstrap driver but as I'm powering the IR2101 driver with 12V, I should have no problem.
Below you can see my driver for the new schematic that I have not testes yet. I've added a ceramic low ESR 3.3uF cap. for bootstrap that should be more than enough. But I'm afraid of it because of my old design problems.
Questions:
Thanks in advance
The Motor should not drive more than 10A @ 24V and the mosfets are the FQP50N06L.
The motor has HALL sensors for commutation control so BEMF is not monitored.
The motor speed is controlled using a 20KHz PWM on bottom FETs.
I designed the driver of each half-bridge using an IR2101.
For the High Side of the bridge the IR2101 uses a diode and a capacitor to do the trick to switch the MOSFET gate.
The only problem is that I'm with problems to calculate and understand exactly this bootstrap capacitor.
Two years ago I designed a BLDC controller using an IC similar to the IR2101. I used the IR2110 which is "stronger" but not needed in this case. Without knowing the capacitor valule I started to add one electrolytic capacitor of 1uF. The motor didn't worked. Then I replaced by a 10uF capacitor. The motor started to spin but very weak. Then I replaced by a 47uF capacitor. The motor worked but when I added some load the commutation were "skiping" then I replaced again the capacitors by a 100uF. This way the motor works always and with a great torque.
Two problems:
1 -
The MOSFET has a low gate charge requirement. In my opinion 100uF for this is an insane value.
According to my calculations the minimum value would be about 300nF.
I'm using a common electrolytic capacitor. The ESR of the capacitor maybe the problem ?
2 -
Motor doesn't start. I need to manually start it, make it spin a little (half a turn) to it start spinning.
That seems to be happening because the bootstrap capacitor too. No enough charge on it.
Is there any special procedure to start a BLDC motor driven by this simple bootstrap strategy ?
I guess not because I saw a lot of schematics on the internet with nothing special...
Some used an additional power supply of -15V to supply the bootstrap driver but as I'm powering the IR2101 driver with 12V, I should have no problem.
Below you can see my driver for the new schematic that I have not testes yet. I've added a ceramic low ESR 3.3uF cap. for bootstrap that should be more than enough. But I'm afraid of it because of my old design problems.
Questions:
- Why I got a weird behavior using a common capacitor with IR2110 and I needed up to 100uF ?
- Why motor can't start spinning without a manual help ?
- My new schematic seems to be correct to drive a motor that will demand up to 24V 10A (Speed control with PWM @ 20KHz) ?
Thanks in advance