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H-Bridge with level shift - MOSFET gate voltage issues?

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Tim Watts
Guest

Wed Jan 02, 2019 10:45 am   



Hi,

Not much good with analogue, would be grateful for any comments on this
please :)

https://www.circuitlab.com/circuit/a5399q386x6h/test-h-bridge/

(or simple picture) https://ibb.co/vh7GRG2

That's half an H-Bridge, we'll have 2 in full H-Bridge configuration
driving a 30V LED garden light string from a Raspberry Pi (details
below). R2/R3 are dummy loads for simulation, please ignore.

Problems:
=========
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this? Something
with a zener maybe?


B) Transistor selection is not final - and TR1 may not work with 3.3V
input (I designed to 5V initially) so we may need a different device or
a buffer gate. I can solve that one way or another - just noting I am
aware of this :)


Many thanks,

Tim

What and why:
=============

Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string.
There'll be 2 of these in full H-Bridge configuration as the LED string
is a mixture of forward and reverse connected substrings. Son wants to
make the patterns more interesting and likes Pi's so it's a fun project.

What I've done so far:

1) Not driving an inductive load, so lose the flywheel diodes common in
motor drives;

2) I've tried it in CircuitLab with and without the TR2/3 driver stage.
The switching speed and power dissipation is a lot better with that
stage at higher switching rates. Whilst we probably won't drive this
much over 1kHz, I prefer to not have software errors able to overheat
things and we need small heatsinks as this is going in a waterproof box
on the lawn;

3) Ignore R2/R3 - this is a dummy load for CircuitLab simulation;

4) R4 is a dummy internal resistance for the PSU (otherwise we get
impossibly high switching transients). In reality the 30V supply is a
current limited switch mode SELV** PSU.

** Separated Extra Low Voltage - UK designation for a "safe to touch"
PSU for wet applications



--
Email does not work


Guest

Wed Jan 02, 2019 5:45 pm   



You need to redesign it, no good with oveestressing Vgs

Use level shift with capacitive method, if you have guaranteed minimum duty cycle

Otherwise, add gate to upper part for reshaping of the drive signal

Cheers

Klaus

John Larkin
Guest

Wed Jan 02, 2019 6:45 pm   



On Wed, 2 Jan 2019 08:46:59 +0000, Tim Watts <tw_at_example.com> wrote:

Quote:
Hi,

Not much good with analogue, would be grateful for any comments on this
please :)

https://www.circuitlab.com/circuit/a5399q386x6h/test-h-bridge/

(or simple picture) https://ibb.co/vh7GRG2

That's half an H-Bridge, we'll have 2 in full H-Bridge configuration
driving a 30V LED garden light string from a Raspberry Pi (details
below). R2/R3 are dummy loads for simulation, please ignore.

Problems:
=========
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this? Something
with a zener maybe?


B) Transistor selection is not final - and TR1 may not work with 3.3V
input (I designed to 5V initially) so we may need a different device or
a buffer gate. I can solve that one way or another - just noting I am
aware of this :)


Many thanks,

Tim

What and why:
=============

Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string.
There'll be 2 of these in full H-Bridge configuration as the LED string
is a mixture of forward and reverse connected substrings. Son wants to
make the patterns more interesting and likes Pi's so it's a fun project.

What I've done so far:

1) Not driving an inductive load, so lose the flywheel diodes common in
motor drives;

2) I've tried it in CircuitLab with and without the TR2/3 driver stage.
The switching speed and power dissipation is a lot better with that
stage at higher switching rates. Whilst we probably won't drive this
much over 1kHz, I prefer to not have software errors able to overheat
things and we need small heatsinks as this is going in a waterproof box
on the lawn;

3) Ignore R2/R3 - this is a dummy load for CircuitLab simulation;

4) R4 is a dummy internal resistance for the PSU (otherwise we get
impossibly high switching transients). In reality the 30V supply is a
current limited switch mode SELV** PSU.

** Separated Extra Low Voltage - UK designation for a "safe to touch"
PSU for wet applications


You could just buy a driver chip, IXDN602 or LTC4442 or one of those.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Lasse Langwadt Christense
Guest

Wed Jan 02, 2019 6:45 pm   



maximum load current?

Tim Watts
Guest

Wed Jan 02, 2019 7:45 pm   



On 02/01/2019 17:36, John Larkin wrote:
Quote:

You could just buy a driver chip, IXDN602 or LTC4442 or one of those.


Wow - I can buy that for little over a quid (first one) - a lot less
than the parts to do it the "hard way")

2A, 35V - that's more than enough.

Thanks very much :)

--
Email does not work

Tim Watts
Guest

Wed Jan 02, 2019 7:45 pm   



On 02/01/2019 17:32, Lasse Langwadt Christensen wrote:
Quote:
maximum load current?



Oh - about 1A tops

--
Email does not work

John Larkin
Guest

Wed Jan 02, 2019 8:45 pm   



On Wed, 2 Jan 2019 18:27:22 +0000, Tim Watts <tw_at_example.com> wrote:

Quote:
On 02/01/2019 17:36, John Larkin wrote:

You could just buy a driver chip, IXDN602 or LTC4442 or one of those.

Wow - I can buy that for little over a quid (first one) - a lot less
than the parts to do it the "hard way")

2A, 35V - that's more than enough.

Thanks very much Smile


It's fun to design stuff, but then some jerk usually comes along and
designs a cheap IC. So we have to move up the abstraction stack.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Lasse Langwadt Christense
Guest

Wed Jan 02, 2019 9:45 pm   



onsdag den 2. januar 2019 kl. 19.27.29 UTC+1 skrev Tim Watts:
Quote:
On 02/01/2019 17:36, John Larkin wrote:

You could just buy a driver chip, IXDN602 or LTC4442 or one of those.

Wow - I can buy that for little over a quid (first one) - a lot less
than the parts to do it the "hard way")

2A, 35V - that's more than enough.

Thanks very much Smile


better read those specs carefully, the output resistance of the first one
will dissipate several watt at 1A, that's a lot for such a small package.

for something simple, old, and cheap how about L298

John Larkin
Guest

Wed Jan 02, 2019 10:45 pm   



On Wed, 2 Jan 2019 12:30:12 -0800 (PST), Lasse Langwadt Christensen
<langwadt_at_fonz.dk> wrote:

Quote:
onsdag den 2. januar 2019 kl. 19.27.29 UTC+1 skrev Tim Watts:
On 02/01/2019 17:36, John Larkin wrote:

You could just buy a driver chip, IXDN602 or LTC4442 or one of those.

Wow - I can buy that for little over a quid (first one) - a lot less
than the parts to do it the "hard way")

2A, 35V - that's more than enough.

Thanks very much :)

better read those specs carefully, the output resistance of the first one
will dissipate several watt at 1A, that's a lot for such a small package.

for something simple, old, and cheap how about L298



Right, calculate the dissipation. Those are intended to be mosfet gate
drivers, but often work fine as load drivers. One can parallel
sections for more current.

The garden lights may be LEDs that don't need a lot of current.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Lasse Langwadt Christense
Guest

Thu Jan 03, 2019 2:45 am   



torsdag den 3. januar 2019 kl. 01.59.08 UTC+1 skrev Clifford Heath:
Quote:
On 2/1/19 7:46 pm, Tim Watts wrote:
(or simple picture) https://ibb.co/vh7GRG2
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this?

You need a high-side driver and/or level shifter.
Don't try to put 30V on the gates.

What and why:
Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string.

HIP4081 is not a bad choice - just add the external MOSFETs.


that would major overkill for a few amps at a few kHz

John Larkin
Guest

Thu Jan 03, 2019 2:45 am   



On Thu, 3 Jan 2019 11:59:00 +1100, Clifford Heath <no.spam_at_please.net>
wrote:

Quote:
On 2/1/19 7:46 pm, Tim Watts wrote:
(or simple picture) https://ibb.co/vh7GRG2
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this?

You need a high-side driver and/or level shifter.
Don't try to put 30V on the gates.


In real life it would be OK. Most fets blow out around 75 volts.

But the resistors will dissipate about 5 watts each, more if the input
ever sticks high or low.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Clifford Heath
Guest

Thu Jan 03, 2019 2:45 am   



On 2/1/19 7:46 pm, Tim Watts wrote:
Quote:
(or simple picture) https://ibb.co/vh7GRG2
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this?


You need a high-side driver and/or level shifter.
Don't try to put 30V on the gates.

Quote:
What and why:
Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string.


HIP4081 is not a bad choice - just add the external MOSFETs.

Clifford Heath.

Clifford Heath
Guest

Thu Jan 03, 2019 2:45 am   



On 3/1/19 12:22 pm, Lasse Langwadt Christensen wrote:
Quote:
torsdag den 3. januar 2019 kl. 01.59.08 UTC+1 skrev Clifford Heath:
On 2/1/19 7:46 pm, Tim Watts wrote:
(or simple picture) https://ibb.co/vh7GRG2
A) It's possible I might be exceeding gate-source voltages on TR2 or TR4
(TR4 max g-d V is 20V). Is there a standard solution to this?

You need a high-side driver and/or level shifter.
Don't try to put 30V on the gates.

What and why:
Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string.

HIP4081 is not a bad choice - just add the external MOSFETs.

that would major overkill for a few amps at a few kHz


He's only gonna build two, not 2 million. The price doesn't matter, so
why not overkill? Anyhow they're only a couple of bucks at Aliexpress.

Tim Watts
Guest

Thu Jan 03, 2019 11:45 am   



On 03/01/2019 00:59, Clifford Heath wrote:
Quote:
On 2/1/19 7:46 pm, Tim Watts wrote:
(or simple picture) https://ibb.co/vh7GRG2
A) It's possible I might be exceeding gate-source voltages on TR2 or
TR4 (TR4 max g-d V is 20V). Is there a standard solution to this?

You need a high-side driver and/or level shifter.
Don't try to put 30V on the gates.


TR1 was supposed to be the level shifter - or at least that's what I was
led to believe in the various "H-Bridge for dummies" guides I found...

I did see some warnings about gate voltages and some vague mention of
zeners but I did not see a practical example anywhere and I can't quite
see how to protect the gates but leave them functional.


I'd like to know how to solve this, even if I use one of the many
excellent driver chips that have been suggested.

We might still do it the "hard way" as this is a project for education
(my son and clearly me too ) rather than solving a real world problem.


========

After that, we're be on to "NeoPixel" strings to complement these lights
and have some real fun :)

I envisage a number of PiZeros in small waterproof boxes local to each
string set, maybe even as much as one high quality 50V DC SELV supply
back in the shed to keep the current and volt drops down (but still
safe) and local DC-DC converter modules in the boxes for both the 30V
lights and the NeoPixel strings. I'll knock up a simple client-server
protocol over WiFi. We'll deploy everything at Haloween next time for
fun and keep over Xmas. The NeoPixels at least won't need drivers (well
maybe a trivial 3.3-5V shifter - I forgot the data bus specs).


We're not planning a lightshow, just some tasteful fun!



--
Email does not work


Guest

Thu Jan 03, 2019 1:45 pm   



The full 30V goes into the stages of the complementary pairs from TR1, so all the transistors except TR1 will have Vgs max violation

Cheers

Klaus

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