# Bootstrap diode, capacitor and resistor for Mosfet gate drive

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#### 123duermed

##### New Member

Hello

I have some questions relating to bootstrapping. If my max voltage on drain would be 300V, then the Vb would be 300V + VCC. So if I have thought right, I need a diode that can hold over 300V.

The voltage over the capacitor will alway be Vcc? because when Vs increase, Vb will increase proportional with Vs. So I just need a capacitors that withstand Vcc
I have understand that 0.1uF is normal, but is this small enough for 50k Hz switching?

How do I decide the resistor as well?

#### kubeek

##### Well-Known Member
I have some questions relating to bootstrapping. If my max voltage on drain would be 300V, then the Vb would be 300V + VCC. So if I have thought right, I need a diode that can hold over 300V.
Yes, but better add 50% safety margin to that.
The voltage over the capacitor will alway be Vcc? because when Vs increase, Vb will increase proportional with Vs. So I just need a capacitors that withstand Vcc
I have understand that 0.1uF is normal, but is this small enough for 50k Hz switching?
Yes, but note that the output voltage needs to drop to zero to recharge the bootstrap cap. So for example if you were feeding a large capacitor with light load, the voltage might not drop low enough.

How do I decide the resistor as well?
This is typically around 10-22ohm.

#### ronsimpson

##### Well-Known Member
The cap only sees Vcc. 0.1 is typical. What does the data sheet say?
The diode sees 300V so use a 500V part. (fast!) not a 1n4007
The resistor is complicated. Start out with 10 ohms.

#### simonbramble

##### Active Member
The capacitor is providing the supply voltage for the high side buffer that switches on the top FET. You need this voltage to hold steady when the top FET switches on, therefore you need this capacitance to be at least 10x the value of the gate-source capacitance of the FET. Large FETs are approx 2000pF, so anything greater than 20nF would be good. 100nF would be better.

personally you might not need a gate resistor unless you notice ringing at the gate (or source) pin of the FET. A gate resistance will only slow down the turn on time of the FET and increase the heat dissipation in it. it depends what the datasheet says

#### 123duermed

##### New Member

So I have order a IR2117. And I planing to use a STTH100W06C diode. It's trr = 55ns and max tfr=200ns. Is this fast enough?

I'm also planing to use the Mosfet STW88N65M5. This mosfet has a Ciss of 8825 pF. So 100nF (0.1 uF) would be just enough?
I have a 0.1uF that can withstand 50V. The mosfets Vgs is max 5V, but I am planing on supplying the driver with 20V. So Is the caps good enough?

#### kubeek

##### Well-Known Member
The mosfets Vgs is max 5V, but I am planing on supplying the driver with 20V. So Is the caps good enough?
Actually the Vgsmax is +/-25V, so yes you will be safe there. I think the switching times will give you no trouble, 50khz is a fairly low frequency.

#### simonbramble

##### Active Member
The VGS threshold voltage is measured when the Drain Source JUST starts to conduct (250uA). If you want to pass more, you will need more gate-source voltage - see the graph. 6V VGS will only get you 25A through the FET. However, 20V is easily enough

#### 123duermed

##### New Member
Do I need a polarized bootstrap caps? Is that a must?

#### crutschow

##### Well-Known Member
Do I need a polarized bootstrap caps? Is that a must?
No, it can be either. There never is a requirement for a polarized capacitor, it's just that larger capacitors are often electrolytic and that type is inherently polarized.

#### 123duermed

##### New Member
Do I need a Rload in my buck regulator when I'm testing for the bootstrapping to work? Since the caps is supposed to charge from 0V. And when i don't have any load, the inductor will not supply any thing with voltage, and then not give me 0v at the Source gate on the mosfet (Vs)