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Forum Index : Electronics : Driving a TLP350 From the Nanoverter

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InPhase

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Joined: 15/12/2020
Location: United States
Posts: 178
Posted: 09:01pm 19 Mar 2021
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I guess I don't fully understand the bootstrap circuit used on the floating high side supply from the IRF21844 ICs on the Nanoverter. So I don't want to blow the TLP350 opto gate driver with a higher voltage. Any insight to clarify how to interface these effectively is much appreciated.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 10:32pm 19 Mar 2021
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Just copy the circuit shown in the IRF21844 datasheet.
https://www.alldatasheet.com/datasheet-pdf/pdf/82797/IRF/IR21844.html

When the lower mosfet is turned on, the source of the upper mosfet is pulled right down to ground.
That causes the diode between the +12v supply and terminal HO to conduct, and charge up the capacitor connected between HO and Vs to +12v.

As the mosfets are turning on and off continuously, the capacitor keeps being charged up to +12v, and that powers the upper gate driver circuit.

Make sure your diode is a a high voltage fast recovery type, UF4007 is recommended.
Cheers,  Tony.
 
InPhase

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Joined: 15/12/2020
Location: United States
Posts: 178
Posted: 02:32am 20 Mar 2021
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  Warpspeed said  Just copy the circuit shown in the IRF21844 datasheet.
https://www.alldatasheet.com/datasheet-pdf/pdf/82797/IRF/IR21844.html

When the lower mosfet is turned on, the source of the upper mosfet is pulled right down to ground.
That causes the diode between the +12v supply and terminal HO to conduct, and charge up the capacitor connected between HO and Vs to +12v.

As the mosfets are turning on and off continuously, the capacitor keeps being charged up to +12v, and that powers the upper gate driver circuit.

Make sure your diode is a a high voltage fast recovery type, UF4007 is recommended.


Thanks Tony. But I already have a Nanoverter with the chips and bootstrap. It is intended to connect directly to the gate(s) of the bridge MOSFETs. Instead, I have an H-bridge with TLP350 optocouplers and isolated gate power supplies. I understand that the bootstrap creates a almost double the chip supply voltage to drive a high side FET. My question is if this voltage can be used to drive the optocouplers or will I need to do a rain dance or some kind of voodoo to put them together.
 
poida

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Joined: 02/02/2017
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Posts: 1418
Posted: 04:14am 20 Mar 2021
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The nanoverter, using IR2184 or IR21844 chips works very well with the
Madness 6kW powerboard. Why this may be interesting is the powerboard uses
TIP41 and TIP42 transistors in a totem pole to drive the MOSFET gates.
It uses a bootstraped supply as well.

So we have the IR21844 providing low side drive and high side drive.
The high side FET sources are connected to the VS pin of the IR21844.

Then on the powerboard we have also a bootstrap to provide the (high side FET source referenced) supply for the high side totem pole drive.

I think you could get a working thing with the TLP350. Maybe a 10uF cap from 12V to H.S. FET source and the UF4007 diode for bootstrap. The opto input would be also
a 12V or so signal, referenced again to the H.S. FET source.
This would be the HO output of the IR21844.

I would have a series current limit resistor on the opto input, to make
about 15mA at 15V. (check with TLP350 docs of course)
wronger than a phone book full of wrong phone numbers
 
wiseguy

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Joined: 21/06/2018
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Posts: 1156
Posted: 02:37pm 22 Mar 2021
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  InPhase said  
Instead, I have an H-bridge with TLP350 optocouplers and isolated gate power supplies. I understand that the bootstrap creates a almost double the chip supply voltage to drive a high side FET. My question is if this voltage can be used to drive the optocouplers or will I need to do a rain dance or some kind of voodoo to put them together.


I like the optocouplers and (separate?) isolated gate power supplies, good practice simple to implement and use.

I'm not sure though that you fully follow the bootstrap operation, so for the benefit of others too I will try to explain it more fully.
The bootstrap is a bit of a weird concept but it doesn't double the voltage.

As Tony eluded to, when the lower FET is on it charges the bootstrap capacitor to the power supply voltage that is used for the lower FET gate drives.

So if you're using 15V for the lower FET drives the bootstrap capacitor is charged to 15V less the bootstrap diode or ~ 14.3V. The negative side of the bootstrap capacitor is typically tied to the Upper FETs source.

When the Lower FET is on the upper FETs source is essentially at ground (or near as dammit), hence the bootstrap capacitor is now charged up. When the Lower FET turns off and the upper FET turns on, regardless of whether the bridge upper supply voltage is 24V or 400V the source of the upper FET will become the upper rail (assume a FET is essentially a short when turned on) and the source will be at 24V (or 400V).

But the + of the bootstrap capacitor will still be 14.3V higher than the Upper FETs source hence when it is used to drive the Gate of the upper FET it can easily keep the upper FET enhanced (on) with its 14.3V of charge.

The size of the bootstrap capacitor has a few variables that affect its size though, the frequency of switching, the gate tie down resistor (often ~ 10K) the gate input capacitance & miller capacitance it has to drive and if you use an optocoupler like the TLP350 its minimum idling current, which is rather high at ~ 10mA max.

Without doing any sums, gut feel tells me the bootstrap capacitor will need to be much larger than normal.  I cant say I have ever seen a bootstrap power supply used to power an Opto FET driver such as the TLP350 - I would advise using the separate isolated supplies and thereby avoid the vodoo & rain dance implications. You only have to make 1 mistake in your maths and you blow whatever savings you might have made with the bootstrap capacitor approach.
Edited 2021-03-23 00:40 by wiseguy
If at first you dont succeed, I suggest you avoid sky diving....
Cheers Mike
 
InPhase

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Joined: 15/12/2020
Location: United States
Posts: 178
Posted: 04:43pm 22 Mar 2021
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  wiseguy said  
  InPhase said  
Instead, I have an H-bridge with TLP350 optocouplers and isolated gate power supplies. I understand that the bootstrap creates a almost double the chip supply voltage to drive a high side FET. My question is if this voltage can be used to drive the optocouplers or will I need to do a rain dance or some kind of voodoo to put them together.


I like the optocouplers and (separate?) isolated gate power supplies, good practice simple to implement and use.

I'm not sure though that you fully follow the bootstrap operation, so for the benefit of others too I will try to explain it more fully.
The bootstrap is a bit of a weird concept but it doesn't double the voltage.

As Tony eluded to, when the lower FET is on it charges the bootstrap capacitor to the power supply voltage that is used for the lower FET gate drives.

So if you're using 15V for the lower FET drives the bootstrap capacitor is charged to 15V less the bootstrap diode or ~ 14.3V. The negative side of the bootstrap capacitor is typically tied to the Upper FETs source.

When the Lower FET is on the upper FETs source is essentially at ground (or near as dammit), hence the bootstrap capacitor is now charged up. When the Lower FET turns off and the upper FET turns on, regardless of whether the bridge upper supply voltage is 24V or 400V the source of the upper FET will become the upper rail (assume a FET is essentially a short when turned on) and the source will be at 24V (or 400V).

But the + of the bootstrap capacitor will still be 14.3V higher than the Upper FETs source hence when it is used to drive the Gate of the upper FET it can easily keep the upper FET enhanced (on) with its 14.3V of charge.

The size of the bootstrap capacitor has a few variables that affect its size though, the frequency of switching, the gate tie down resistor (often ~ 10K) the gate input capacitance & miller capacitance it has to drive and if you use an optocoupler like the TLP350 its minimum idling current, which is rather high at ~ 10mA max.

Without doing any sums, gut feel tells me the bootstrap capacitor will need to be much larger than normal.  I cant say I have ever seen a bootstrap power supply used to power an Opto FET driver such as the TLP350 - I would advise using the separate isolated supplies and thereby avoid the vodoo & rain dance implications. You only have to make 1 mistake in your maths and you blow whatever savings you might have made with the bootstrap capacitor approach.


Thanks, I see how that works now. I also studied the functional diagram for the IR21844 driver IC. I thought I might be able to feed the Vb pin from the isolated supply since the Vs pin is already connected to the upper FETs source and hence the negative of the isolated supply. I could get rid of the bootstrap components completely... Am I thinking about that correctly?
 
wiseguy

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Joined: 21/06/2018
Location: Australia
Posts: 1156
Posted: 10:21pm 22 Mar 2021
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I like to see a reference schematic to ensure we are both talking apples. A bootstrap supply is used in lieu of an isolated supply (the bootstrap function is a kind of cheap isolated supply - isolated from the lower FETs ground or negative supply).

When using a dedicated separate isolated supply yes it just connects between the Vb pin and Vs pin & get rid of the original bootstrap diode. Note: Vs must still be tied to the upper FETs source.

The only remaining component of the original bootstrap setup to keep is the low impedance ceramic capacitor close to the VB & VS pins (Typically 0.47u - 1u).

If the isolated supply does not have an electrolytic in sight, as per the little murata 1 & 2W types, also use a 4.7u - 10u electrolytic in parallel with the ceramic.
I often use a tantalum type for this, but a good quality 105 degree type electrolytic is also commonly used.

When using the little ali express 100 - 240VAC to 15V types they often already have the electrolytic on their PCB. (the input voltage to these often function fine from ~35VDC upwards for gate driving purposes).
Edited 2021-03-23 14:48 by wiseguy
If at first you dont succeed, I suggest you avoid sky diving....
Cheers Mike
 
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