Improving Voltage Drop In Low-Voltage H-Bridge Motor Drivers

Core Issue: Voltage Drop Across Power Devices

The core technical issue in H-bridge motor drivers that influences voltage regulation is the unavoidable voltage drop that occurs across the power devices during current flow. This voltage drop is described by Ohm’s law as:

Voltage Drop = Load Current x On-State Resistance of Power Devices

This voltage drop directly reduces the voltage supplied to the motor, leading to poorer speed regulation and performance. Mitigating this drop is essential for precision motor control applications requiring tight voltage regulation such as CNC machines, robots, and drones.

Table of Contents

Key Factors Influencing Voltage Drop

Driver Topology and Configuration

The specific topology and configuration of the H-bridge and its associated components impact how current flows through the power devices, influencing the voltage drop:

Half-bridge vs full bridge topology – Half-bridge subjects devices to higher currents
Use of synchronous rectification – Can reduce current flow through high-side devices
PWM switching frequency – Higher frequency increases switching losses
Gate drive strength – Stronger gate drive reduces Rds(on) of devices

Device On-State Resistance

The on-state resistance (Rds(on)) of the metal-oxide-semiconductor field effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs) used directly impacts the voltage drop according to Ohm’s law. Lower resistance devices proportionally reduce this drop.

Load Current Magnitude

Higher load currents induce larger voltage drops across power devices. Accurately determining maximum load currents is essential to simulating and predicting the worst-case voltage drop.

Mitigation Strategies

Optimizing PCB Layout

Minimizing Parasitic Inductance

Careful component placement and routing techniques minimize parasitic inductance in the high current motor drive loop. This keeps transient voltage spikes to a minimum during switching.

Use dedicated power and signal ground planes
Place decoupling capacitors close to IC power pins
Route high-current traces over solid ground planes

Careful Decoupling

Proper decoupling capacitor selection and placement supplies charge to stabilize voltage during load transients. This prevents momentary voltage drops at the driver and motor.

Use low ESR ceramic capacitors for high frequency decoupling
Additional bulk capacitance electrolytics handle lower frequency transients
Calculate required capacitance based on load current and voltage tolerance

Implementing Closed-Loop Control

Compensating for Voltage Drop With Feedback

Adding a voltage feedback loop with an ADC and microcontroller enables real-time compensation of voltage drops by adjusting the PWM duty cycle to maintain regulation at the motor terminals.

Feedback mechanism measures actual motor voltage
Error amplitude modulates PWM duty cycle to compensate
Regulates motor voltage and increases stiffness

Example Control Code

Here is example C code for a basic PI control loop to regulate motor voltage and overcome any drops:

float v_error; // Voltage error 
float v_integ; // Integral term

void control_loop() {

  v_meas = read_adc(); // Measure actual voltage
  
  v_error = v_ref - v_meas; // Voltage error
  
  v_integ += v_error; // Accumulate integral 
  v_integ = constrain(v_integ, -0.5, 0.5); // Limit
  
  duty = kp*v_error + ki*v_integ; // PI loop equation 
  
  pwm_duty(duty); // Update PWM duty cycle
  
}

The gains kp and ki can be tuned to optimize transient response and stability.

Selecting Components

Choosing FETs With Lower Rds(on)

Carefully selecting MOSFETs and IGBTs with the lowest on-state resistance at the required voltage/current minimizes conduction losses:

GaN devices offer lowest Rds(on) but have higher cost
Multi-die power blocks combine multiple dies in parallel
Balance Rds(on) against gate charge and switching losses

Trading Off Cost vs Performance

A cost vs performance analysis should guide component selection. More expensive low-Rds(on) devices can pay for themselves through improved efficiency and voltage regulation.

Use device datasheets to model losses and efficiency
Estimate cost savings from reduced power dissipation
Consider both BOM cost and lifetime power usage

Achieving Under 1% Voltage Regulation

Combining Above Techniques

Employing a combination of layout considerations, closed-loop control, and low-Rds(on) components enables exceptional voltage regulation under 1%:

MOSFETs selected for sub 2 milliohm Rds(on)
4-layer PCB with dual ground planes
PID voltage feedback control loop
Tuned compensation network

This produces stiff and responsive motor voltage control with minimal drop, even over wide load variations.

Meeting Requirements of Motor System

Achieving sub 1% voltage regulation meets the precision demands of sensitive mechatronic systems, providing headroom for:

Increased torque and top speed in robot arms
Improved velocity stability in CNC machines
Enhanced maneuverability and stiffness in aerial drones

Precision voltage control is imperative to fully utilize the performance of modern servo motors.