H-Bridge Inverter

Goal

Basic functionality of an H-Bridge Inverter (12V DC --> 230V AC)

Overview

1. Microcontroller (Raspi Pico) generates a sine PWM as a control signal
2. Sine PWM goes into a gate driver (Bootstrap)
3. Bootstrap (one for each side) controls H-Bridge MOSFETs --> sine_wave AC

Fig.1 - Simple H-Bridge

Code

• based on sPWM_Basic/sPWM_Basic.ino

Flowchart

Fig.2 - Flowchart Code

Output

Fig.3 - Sine PWM Output

Hardware

Why Bootstrap Circuit?

• source of high side MOSFET floats between V_AC (here: 12V AC) --> V_GS at the high side MOSFET must be high enough to safely switch on/off the MOSFET
• if low side MOSFET on, Bootstrap capacitor is chraged up to V_DC (here: 12V DC, voltage drop over bootstrap diode negligible)
• if high side MOSFET is switched on, the capacitor delivers its voltage to the control pin of the high side MOSFET

Fig.4 - Bootstrap CIrcuit

Circuit Design

• Infineon IR2104 was used
• Dimensioning based on TI Application Note
• MOSFETs: IRFB7537

Bootstrap Capacitor

$C_{boot} >= \frac{Q_{total}}{\Delta V_{HB}} = 183.75nF$ (Rule of thumb: 10 times gate capacitance leads to $190nF$) --> $C_{boot} = 200 nF$

$Q_{total} = Q_G + I_{HBS} \cdot \frac{D_{max}}{f_{sw}} + \frac{I_{HB}}{f_{sw}} $

• $Q_G = 210nC$ from Data Sheet MOSFET

• $I_{HBS} = 50 \mu A$ from Data Sheet Gate Driver

• $D_{max} = 1$ (will be slightly lower beacause of 520ns Dead time, 100% assumed for conservative caluclation)

• $f_{sw} = 10kHz$

• $I_ {HB} = 55 \mu A$ from Data Sheet Gate Driver

$\Delta V_{HB} = V_ {DD} − V_ {DH} − V_ {HBL} = 1.2V $

• $V_ {DD} = V_{DC} = 12V$

• $V_ {DH} = 1V $ from Data Sheet 1N4148

• $V_{HBL} = 9.8V$ from Data Sheet Gate Driver

Bootstrap Resistor

$R_{boot} = \frac{V_{DD} - V_{Boot,Diode}}{I_{peak}} = 5.5 \Omega$ --> $5.6 \Omega$

• $V_{DD} = V_{DC} = 12V$

• $V_{Boot,Diode} = V_ {DH} = 1V $ from Data Sheet 1N4148

• I_{peak} = I_{FSM} = 2A from Data Sheet 1N4148

Gate Resistors

$R_{G,HS} = \frac{V_{Gate}}{I_{o+}} = 92 \Omega$ --> $91 \Omega$

• $V_{Gate} = V_{DC} = 12V$
• $I_{o+} >= 130mA $ from Data Sheet Gate Driver

$R_{G,LS} = \frac{V_{Gate}}{I_{o-}} = 44 \Omega$ --> $47 \Omega$

• $V_{Gate} = V_{DC} = 12V$
• $I_{o+} >= 270mA $ from Data Sheet Gate Driver

Output Filter

• second order passive lowpass filter (LC filter)
• Cutoff frequency $f_g = \frac{1}{2 \pi \cdot \sqrt{LC}}$ (see ElectronicBase.net)
• Cap chosen based on availabilty in store: $C_{Filter} = 10 \mu F$ (use film cap, not a polarized one)
• $L_{Filter} = 100mH$ --> $f_g = 159.2Hz$

Results

The higher the resolution of the sine PWM (pwm_periods & scaler, see code) the clearer the sine wave output gets. Since the performance of the C code is better than the Micropython code (see below), the sine wave from the C code has less ripples. The amplitude is relatively low at the moment since there is no output voltage regulation yet.

Fig.5 -Resulting Sine Wave with Micropython Code

Fig.6 -Resulting Sine Wave with C Code

Performance

Disclaimer: I know Micropython is not made for high performance applications, but rather for rapid prototyping and easy debugging. I still wanted to see how far I can get with Micropython and compare it to C. Also the C code can become much faster by shorten the timer callback, using bit operations and more. I am looking forward to any improvement suggestions :)

Language	Code Version	$f_{sin,set}$ in Hz	$f_{sin,real}$ in Hz	$f_{switch,real}$ in kHz
MicroPython	26/12/2024	50	50	5
MicroPython	26/12/2024	100	100	10
MicroPython	26/12/2024	500	105.3	10.5
C	02/01/2025	50	50	5
C	02/01/2025	100	100	10
C	02/01/2025	500	444.4	44.4

ToDos

• shorten timer callback
• add output voltage regulation
• (if inductive load: add external flyback diodes in parallel to MOSFETs)