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Half-bridge/gate drive heeeeelp!

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Michael Terrell
Guest

Sun May 03, 2020 3:45 am   



On Saturday, May 2, 2020 at 9:37:48 PM UTC-4, Jon Elson wrote:
Quote:
bitrex wrote:

Here's the situation in simplified schematic form, generate a
high-efficiency sine wave at around 3-5MHz putting out about 5 watts
into resistive load:
You want to generate a 5 MHz sine wave with PWM? What kind of PWM frequency
would you need for that? 50 MHz?

Look up the gate driver specs for dissipation, and you will likely see NONE
of them can handle charging and discharging a FET gate at much over 100 KHz
rates without running very hot.

One other thing I ran into while designing full bridge PWM amps was that the
FET body diodes take microseconds to turn on, but when the high-side
transistor cuts off, current flowing out through the inductor needs to be
supplied from somewhere. The body diodes can easily sit there for several
microseconds with a forward bias of 7 - 12 V without conducting at all.
So, I had to add an ultrafast diode across the low-side transistor to
prevent the common node from going too far negative. Many gate drivers can
only handle the common node going so far below ground before they pop.


Harris Broadcast has built modular AM transmitters that use PWM for modulation, The BCB models that I've seen were about 250 W per module, with 20 modules for the 5KW transmitter used by WQBQ in Leesburg Florida. It was a first generation transmitter digital. It had the optional Microprocessor board that monitored all of the modules, and could email the engineer when there was a problem. I understand it was replaced after 20 years with the latest version, because spare parts were hard to locate. The original RF modules used TO-3 packages that quickly became obsolete, even though Harris Semiconductor had manufactured them. A complex array of dividers and combiners allowed a failed module to be turned off until it could be replaced. That transmitter replaced a worn out Gates 5KW tube based BC5000 series that I removed from the building to make room for them to move their studios back to the transmitter site. It was full of old oil capacitors filled with PCB based transformer oil. It was five times the size of the new transmitter, and it printed out the required FCC data log, so no on site operator was needed. That Gates transmitter went to another local station for spare parts. (WLBE)

bitrex
Guest

Sun May 03, 2020 4:45 am   



On 5/2/2020 9:37 PM, Jon Elson wrote:
Quote:
bitrex wrote:

Here's the situation in simplified schematic form, generate a
high-efficiency sine wave at around 3-5MHz putting out about 5 watts
into resistive load:
You want to generate a 5 MHz sine wave with PWM? What kind of PWM frequency
would you need for that? 50 MHz?

Look up the gate driver specs for dissipation, and you will likely see NONE
of them can handle charging and discharging a FET gate at much over 100 KHz
rates without running very hot.


You can go much faster than that nowadays, 100kHz isn't even that fast
for a modern buck converter. Pretty slow

Quote:
One other thing I ran into while designing full bridge PWM amps was that the
FET body diodes take microseconds to turn on, but when the high-side
transistor cuts off, current flowing out through the inductor needs to be
supplied from somewhere. The body diodes can easily sit there for several
microseconds with a forward bias of 7 - 12 V without conducting at all.
So, I had to add an ultrafast diode across the low-side transistor to
prevent the common node from going too far negative. Many gate drivers can
only handle the common node going so far below ground before they pop.

Jon


50MHz PWM? no, they're being driven directly at the lower frequency,
switching the four PSU rails into an LC filter, like firing pistons in a
4-cylinder engine, consider:

<https://www.dropbox.com/s/tz1zp0d8oy8zkws/IMG_20200501_133832793.jpg?dl=0>

(there should be a cap in line with the inductors to eliminate offset
problems in the absence of some kind of active feedback that keeps the
switching nodes locked together)

No PWM here

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