Capstone 2: AI Smart Power Supply

Specifications

Power Architecture

flowchart LR
    A["USB-PD Input
20V/3A"] --> B["FUSB302
PD Negotiation"]
    B --> C["Input Protection
TVS + Fuse"]
    C --> D["Sync Buck
LM5166"]
    D --> E["Output Stage
LC Filter"]
    E --> F["Output
Terminals"]
    G["STM32G4
MCU"] -->|DAC| D
    G -->|I2C| H["INA219
V/I Sense"]
    H --> F
    G -->|SPI| I["ILI9341
Display"]
    G --> J["Encoder
+ Buttons"]
    style G fill:#3B9797,color:#fff
    style F fill:#BF092F,color:#fff
                            

# Buck converter component selection calculator
# Synchronous buck: LM5166 or TPS54360

# Requirements
V_in_max = 36.0     # Max input voltage (V)
V_out = 12.0        # Target output voltage (V)
I_out = 5.0         # Max output current (A)
f_sw = 500e3        # Switching frequency (Hz)
V_ripple = 0.050    # Max output ripple (V, peak-to-peak)

# Duty cycle
D = V_out / V_in_max

# Inductor selection
delta_IL = 0.3 * I_out  # 30% ripple current
L_min = (V_out * (1 - D)) / (f_sw * delta_IL)

# Output capacitor
C_out_min = delta_IL / (8 * f_sw * V_ripple)

# Input capacitor (RMS current)
I_cin_rms = I_out * (D * (1 - D)) ** 0.5

print("Synchronous Buck Converter Design")
print("=" * 50)
print(f"Input:  {V_in_max}V max")
print(f"Output: {V_out}V @ {I_out}A")
print(f"Freq:   {f_sw/1e3:.0f} kHz")
print(f"\nDuty cycle:     {D:.2%}")
print(f"Inductor:       {L_min*1e6:.1f} µH min (use {round(L_min*1e6/2.2)*2.2:.1f} µH)")
print(f"Ripple current: {delta_IL:.2f} A ({delta_IL/I_out:.0%} of Iout)")
print(f"Output cap:     {C_out_min*1e6:.1f} µF min (use {max(22, round(C_out_min*1e6/10)*10)} µF)")
print(f"Input cap RMS:  {I_cin_rms:.2f} A (use X5R/X7R rated)")
print(f"\nPower: {V_out * I_out:.0f}W, Efficiency est: ~{90 if D > 0.3 else 85}%")
print(f"Loss:  ~{V_out * I_out * 0.1:.1f}W → heat sink required")

Digital Control Loop

/* PID voltage control loop — runs in ADC interrupt
   DAC output adjusts buck converter reference voltage */

#include <stdint.h>

typedef struct {
    float Kp;          /* Proportional gain */
    float Ki;          /* Integral gain */
    float Kd;          /* Derivative gain */
    float setpoint;    /* Target voltage (mV) */
    float integral;    /* Accumulated error */
    float prev_error;  /* Previous error for derivative */
    float output_min;  /* DAC minimum (0) */
    float output_max;  /* DAC maximum (4095 for 12-bit) */
} pid_controller_t;

/* Initialize PID with tuned gains */
pid_controller_t voltage_pid = {
    .Kp = 2.5f,
    .Ki = 0.8f,
    .Kd = 0.1f,
    .setpoint = 5000.0f,  /* 5.000V in mV */
    .integral = 0.0f,
    .prev_error = 0.0f,
    .output_min = 0.0f,
    .output_max = 4095.0f,
};

float pid_update(pid_controller_t *pid, float measurement) {
    float error = pid->setpoint - measurement;

    /* Anti-windup: only integrate if output not saturated */
    pid->integral += error;
    if (pid->integral > pid->output_max) pid->integral = pid->output_max;
    if (pid->integral < pid->output_min) pid->integral = pid->output_min;

    float derivative = error - pid->prev_error;
    pid->prev_error = error;

    float output = pid->Kp * error
                  + pid->Ki * pid->integral
                  + pid->Kd * derivative;

    /* Clamp output to DAC range */
    if (output > pid->output_max) output = pid->output_max;
    if (output < pid->output_min) output = pid->output_min;

    return output;
}

/* Called from ADC interrupt at 100 kHz sample rate */
/*
 * void ADC1_IRQHandler(void) {
 *     float v_sense = adc_read_mv(ADC_CH_VOUT);
 *     float dac_val = pid_update(&voltage_pid, v_sense);
 *     DAC1->DHR12R1 = (uint32_t)dac_val;
 * }
 */

Protection Circuits

Power Supply Spec Sheet Tool

Conclusion

The smart power supply capstone combines analog power electronics (buck converter), precision measurement (INA219), digital control (PID loop), and user interface design into a complete bench instrument. The USB-PD input makes it portable and compatible with modern laptop chargers.