How can I drive a MOSFET gate from a 12 V source to a 24 V pulse for a high-side switch?
Use a dedicated high-side MOSFET gate-driver/charge-pump circuit instead of a discrete transistor pulser; an IR2112 or IR2117 bootstrap driver was suggested for this job [#21660907][#21660908][#21660916] The driver should use bootstrap operation with two diodes and two capacitors, and its gate-drive supply range is about 10–20 V [#21660907][#21660908] One reply points to the IRF application note for a buck converter with a bootstrap high-side gate driver as the correct reference design [#21660914][#21660916] Another practical fix was to increase the bootstrap capacitor to greater than 0.47 µF when the MOSFET was not turning off properly [#21660922] A later test report said that increasing the bootstrap capacitance and shortening the PWM duty cycle improved operation and made the PMA much easier to spin [#21660923] If you still want a fully discrete approach, the thread notes that it is possible but adds complexity and headaches [#21660907]
Hi, I redrew my circuit showing point X is 24 volts. The mosfet source is 12 volts. I am searching for a transistor circuit that will pulse from 12 to 24 volts to dirve the mosfet gate. Comments are welcome.
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Wow! Using a charge pump is the right thing to do, but you made it too complicated. Google, "buck gate drive charge pump."
In any event you'll need a high side driver, better to get IC for that, two diodes, two caps. Since you need a PWM, you can use that for your CP assuming not too much gate charge on the FET, get rid of existing CP. Or just use existing PWM with built in CP, you're done! But if you insist on headaches and perhaps a learning experience do it with all discretes.
Why not use a custom mosfet driver such as an IR2112 supplied by international rectifier Floating channel designed for bootstrap operation • Fully operational to +600V • Tolerant to negative transient voltage dV/dt immune • Gate drive supply range from 10 to 20V • Undervoltage lockout for both channels • 3.3V logic compatible Separate logic supply range from 3.3V to 20V Logic and power ground ±5V offset • CMOS Schmitt-triggered inputs with pull-down • Cycle by cycle edge-triggered shutdown logic
What David sent won't work as is but it's close. Get rid of the lower 1K resistor because the MOSFET must be driven to below say three volts, not 12V. Also you don't need a Baker clamp on the NPN, seriously this is not a power transistor where you have to deal with deep storage and trailing current. Speed isn't an issue. Also the NPN inverts the signal so you'll need a second inversion so use an inverter for that. Otherwise it'll work. You should know that the classic NPN/PNP driver has shoot-through so the driver at 24V will heat up and probably limit the switching to say about 100KHz.
Hi, on page 22 and 23 there seems to be a solution. It looks like Vcc would be replaced by the PMA, but I am not sure. Thank you to everyone for all the suggestions Comments welcome.
Hi, the IR2117 have arrived and I completed all the high current connections of the buck converter. Using a Zener diode as Vcc to power just the nand gate oscillator I was able to illuminate a LED with a quarter turn of the PMA. Not great results but it is a start. I have to test the IR2117 with the oscilloscope to determine if HO is oscillating. I wired the test circuit and used a 12 volt halogen light as a test load and spun the PMA manually. The PMA was very easy to spin until the MOSFET turn on, then I was unable to spin the PMA. I believe the MOSFET is not turning off. My concerns are there is a lot energy stored in the input capacitor and I do not want to harm my oscilloscope. Are there any points I should avoid when testing the circuit????? The chassis ground and each channel ground are a the same common point, is there a safe method to use when testing the circuit with the scope? Comments welcome.
The rectified and filtered output of your three-phase Y is 24 volts correct?
If the center point of the three phase Y is connected to earth ground (same as the oscilloscope AC power ground) that is where you must connect the oscillcope probe ground. This can make it difficult to probe the circuit and get a meaningful measurement.
Or, you can 'float' the oscilloscope by disconnecting the ground prong of the AC plug. I know folks object to this practice but it's common.
If the three phase Y is not grounded (it is floating) to can connect the battery (-) terminal to earth ground and connect the oscillscope probe ground to that point.
Hi, depending on the RPM the output can range from 0 volts to over 60 volts. I found a application note from Fairchild that I am trying to use as a guide line.
I have to read the paper a few times, it is starting to make more sense.
So far the oscillator is working. If I leave the MOSFET out of the circuit and connect Vs of the IR2117 to ground the photo shows input timing signal from 555 timer and output (HO) of IR2117.
When I add the MOSFET the last photo shows the IR2117 will turn on but not off. I am currently working to solve this problem. Comments are welcome and thank you for the help I have been receiving.
Hi, video shows manual operation of PMA and buck converter to a resistive load. The mosfet is turning on during the off cycle of the IR2117. Is there a solution to this problem??? Comments welcome.
A safety margin resistor diode network can be added to the gate, as shown See attached
The purpose of this network is to further delay the turn-on, without affecting the turn-off, thereby inserting some additional dead-time.
The resistor-diode network is also useful in reducing the peak of the current spike during the reverse recovery time.
This has an impact on power losses, as well as dv/dt and EMI.
You can also add a PNP transistor such that the base is attached to the Anode of the diode resistor network , the emitter to the cathode of the diode resistor network and the collector to ground This shuts off the fet completley ensuring that no gate voltage is present during the off cycle
Greetings, I added the larger caps to the boot strap and the ringing is still there but only noticeable at lower RPM’s.
I am still using the 555 timer. I increased the duty cycle to 80% and inverted the output so the duty cycle at the input of the IR2117 is 20% high. I connect a 12 volt 50 watt light and spinning the PMA manually I was able to illuminate the 50 watt light with relative ease. Wow, a shorter duty cycle made all the different. The PMA was much easier to spin and even maintained momentum for a short period when I stopped spinning the PMA.
Without the buck converter I was unable to illuminate the 50 watt load under manual operation.
I am still using the bench power supply to power the IC’s.
Performance has definitely improved.
I still need away to control the duty cycle at different PMA RPM’s and find away to power the IC’s from the battery or PMA. Next step is to try charging the battery. It maybe time to start thinking about adding a micro controller. Thank you for all the suggestions.
✨ The discussion focuses on designing a transistor circuit to pulse a MOSFET gate voltage from 12V to 24V, with the MOSFET source at 12V and the gate requiring a higher voltage drive. Key solutions include using a high-side driver with a bootstrap charge pump, which simplifies the design compared to discrete components. The IR2112 and IR2117 gate driver ICs from International Rectifier are recommended for their floating channel bootstrap operation, wide voltage tolerance, and undervoltage lockout features. Practical issues addressed include ensuring proper gate drive voltage below threshold levels, avoiding shoot-through in transistor drivers, and managing switching frequency limitations due to heating. Additional circuit modifications involve resistor-diode networks and PNP transistors to introduce dead-time and prevent gate voltage during off cycles, reducing EMI and power losses. Testing challenges with oscilloscope grounding and safe measurement practices are discussed. Application notes from International Rectifier and Fairchild Semiconductor provide design guidance, including increasing bootstrap capacitor values to improve performance. Experimental results show improved MOSFET switching and load driving using a 555 timer PWM input with adjusted duty cycles, enabling manual operation of a permanent magnet alternator (PMA) and illumination of resistive loads. Future steps include controlling duty cycle relative to PMA RPM and powering the driver ICs from the battery or PMA output. Generated by the language model.
TL;DR: Use a bootstrap high‑side MOSFET driver (e.g., IR2112/IR2117); "use a custom MOSFET driver such as an IR2112." Typical gate‑drive supply is 10–20 V. [Elektroda, Mark Harrington, post #21660908]
Why it matters: This fixes "how do I drive a high‑side MOSFET from 12 V to 24 V?" safely, for builders of buck converters and PMA chargers.
A gate resistor + diode can add dead‑time and reduce reverse‑recovery current spikes. [Fairchild AN-6076]
How do I pulse a MOSFET gate from 12 V to 24 V on the high side?
Use a high‑side driver IC with a bootstrap supply. The driver’s HO pin rides on the switch node and charges a bootstrap capacitor through a diode, lifting the gate above the source during turn‑on. Devices like IR2112/IR2117 are designed for this and include UVLO and level shifting. Keep the bootstrap loop tight and place the cap close to VB/Vs pins. [IR AN-978]
What is a bootstrap high‑side driver, in simple terms?
It is a MOSFET gate driver that uses a small diode‑capacitor “bootstrap” to create a temporary voltage higher than the MOSFET source. That extra headroom turns on an N‑channel MOSFET on the high side. See the buck chopper example using IR2117 for the block‑level view. [USNA EE320 Gate Driver Supplement]
Do I need a charge pump, or can my PWM handle it?
You can use a driver with a built‑in charge pump or bootstrap and feed it your PWM. As one expert put it, "Using a charge pump is the right thing to do." If your PWM already exists, pair it with a proper high‑side driver and skip discrete charge‑pump experiments. [Elektroda, Earl Albin, post #21660907]
My high‑side MOSFET turns on during the OFF cycle—what’s wrong?
Likely causes include insufficient gate discharge, dv/dt coupling, or too small a bootstrap capacitor. Add a gate resistor‑diode network to slow turn‑on and speed turn‑off, inserting dead‑time. A small PNP can clamp the gate to ground during OFF for certainty. "This shuts off the FET completely." [Elektroda, Mark Harrington, post #21660921]
How large should the bootstrap capacitor be?
Size it to cover gate charge, driver consumption, and leakage. A practical fix from the field is increasing Cboot to ≥0.47 µF, which cures dropout and OFF‑state misbehavior in many cases. Place it close, use low‑ESR/ESL parts, and keep traces short. [IR AN-978]
What gate voltage should I target for ON and reliable OFF?
Drive the gate high within the driver’s 10–20 V range for strong ON. Ensure the gate falls well below threshold for OFF; an expert warns the MOSFET must be pulled under about 3 V rather than left near 12 V. Use proper pull‑downs and discharge paths. [Elektroda, Earl Albin, post #21660913]
How do I probe this safely with an oscilloscope?
If the three‑phase Y center or battery negative is earth‑referenced, connect the probe ground there. Otherwise, establish a single system ground point. Some float the scope by lifting the ground pin, but that carries risk. Keep ground leads short to reduce ringing. [Elektroda, DAVID CUTHBERT, post #21660918]
Is a P‑channel MOSFET an option for the high side here?
Yes. A simple driver works if you switch to a P‑channel MOSFET. The gate then pulls down to turn ON and releases to turn OFF. This can simplify level shifting, but conduction losses are higher than with an N‑channel of similar size. [Elektroda, DAVID CUTHBERT, post #21660909]
How can I add dead‑time and cut reverse‑recovery spikes?
Use a series gate resistor for turn‑on and a parallel diode to bypass it on turn‑off. This delays turn‑on without slowing turn‑off, reducing current spikes during diode reverse‑recovery. It also lowers EMI and switching losses. [Fairchild AN-6076]
Why does a simple NPN/PNP totem driver heat up and limit frequency?
That classic push‑pull has shoot‑through during transitions. At 24 V it can heat the pair and cap switching near 100 kHz. Add proper dead‑time, or use a dedicated MOSFET driver with built‑in control and fast edges. [Elektroda, Earl Albin, post #21660913]
Three‑step: wiring an IR2117 for a buck converter high side
Place the bootstrap diode from VCC to VB and the bootstrap capacitor from VB to VS, close to the IC.
Connect HO to the MOSFET gate, VS to the MOSFET source/switch node, and LO unused for high‑side‑only.
Feed PWM into IN; tie COM to logic ground and decouple VCC well. [USNA EE320 Gate Driver Supplement]
What is a charge pump?
A charge pump is a switch‑capacitor DC/DC circuit that creates a higher or inverted voltage without an inductor. In gate drivers, it maintains gate headroom for long on‑times when bootstrap refresh is limited. [“Charge pump”]
Will tweaking PWM duty help a hand‑spun PMA start lighting a load?
Yes. In tests, inverting to get about 20% high at the driver eased spinning and lit a 12 V/50 W lamp. Lower duty reduced torque drag from the buck stage during startup and improved momentum. [Elektroda, Lauri Koponen, post #21660923]
How should I power the control electronics when the PMA varies 0–60 V?
Use a regulated VCC for logic. A small Zener‑based supply can run an oscillator for bring‑up, but a proper buck or LDO from the battery is better for stability and UVLO margins on the driver IC. [Elektroda, Lauri Koponen, post #21660917]
What high‑voltage headroom do IR211x drivers offer for floating channels?
IR211x devices are built for high‑side operation with large bus voltages. One referenced driver family specifies a floating channel “fully operational to +600 V,” giving ample margin in automotive and industrial bucks. [Elektroda, Mark Harrington, post #21660908]
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