Complete Guide to DSP Feedback Circuit Design for 4kW LLC Converters: From Digital Control to Levane Power HLF4K Practice

I. Introduction: A Paradigm Shift Brought by Digital Control

In previous articles of this series, we have examined the transformer, power switches, rectifier diodes, output capacitors, and analog feedback circuits of a 4kW LLC converter. However, as the requirements for intelligence in tethered drones and smart grids increase, the limitations of traditional analog feedback solutions in remote monitoring, flexible configuration, and fault diagnosis become more apparent.

The introduction of a Digital Signal Processor (DSP) fundamentally changes the design approach of the feedback loop: the output voltage is no longer amplified by a TL431 and an optocoupler; instead, it is sampled by an ADC, computed by a digital compensator (PID), and then drives the power stage via a high‑resolution PWM/PFM module. This architecture not only retains the stability advantages of analog loops but also brings programmability, communication interfaces, and a high level of system integration.

The Levane Power HLF4K series leverages the digital control capability of a DSP to achieve >97% efficiency while providing remote voltage adjustment, status reporting, and intelligent protection. This article systematically describes the design method of a DSP‑based feedback circuit for a 4kW LLC converter, covering hardware architecture, sampling and conditioning, digital compensator design, software protection, and engineering practices used at Levane Power.

II. Hardware Architecture of DSP Feedback Control

2.1 Overall System Solution

There are two common approaches for digital power hardware:

For a 4kW LLC converter, a general‑purpose DSP such as the TI C2000 series is the preferred choice for industrial products due to its powerful floating‑point capability and rich motor‑control peripherals.

2.2 Isolated Voltage Sensing Circuit

In a high‑voltage isolated DC‑DC converter, the DSP is usually placed on the secondary side to directly sample the output voltage and current, and then sends the control signal (PWM or PFM) back to the primary‑side driver through an isolation device.

Recommended solution: resistor divider + direct ADC sampling

• The output voltage Vo is reduced by a resistor divider (e.g., R1 = 24 kΩ, R2 = 1 kΩ) to the ADC input range (0‑3.3 V) of the DSP.

• To suppress high‑frequency noise, add an RC low‑pass filter (cut‑off frequency ≈ 10 kHz) before the ADC pin.

• Because the DSP shares the ground with the secondary side, an expensive isolation amplifier is not required, significantly reducing cost.

Key considerations:

• The divider resistors should be 1% tolerance to maintain output voltage accuracy.

• The sensing traces must stay away from strong interferers such as the transformer and power inductors, and should be routed as a differential pair.

2.3 Current Sensing and Protection Circuit

A 4kW LLC converter requires monitoring two types of current:

• Load current for loop control.

• Resonant current for overcurrent protection.

For overcurrent protection, it is recommended to use the high‑speed analog comparator inside the DSP (e.g., C2000 CMPSS). When the threshold is exceeded, the PWM is shut down in hardware; response time can be less than 100 ns.

2.4 Isolated PWM/PFM Signal Transmission

The control signal generated by the DSP (PFM frequency value or PWM duty cycle) must be transferred to the primary‑side gate driver through an isolation device.

• Option 1: Digital isolator – Use a high‑speed digital isolator (e.g., ISO7240) to transmit the DSP’s PWM signal directly.

• Option 2: Isolated CAN transceiver – If monitoring data also needs to be transmitted, use an isolated CAN transceiver (e.g., ISO1042) to package control commands and status information, reducing the number of isolation channels.

The Levane Power HLF4K series uses the digital isolator solution to ensure real‑time control signal transmission.

III. Digital Loop Compensation Design

3.1 Conversion from Continuous‑Time to Discrete‑Time Domain

Analog feedback loops use op‑amps and RC networks for compensation, while digital loops implement compensation via software algorithms.

Design flow:

1. Extract the small‑signal model of the power stage – Use SIMPLIS or PSIM to simulate the control‑to‑output transfer function GCS of the LLC converter.

2. Design a continuous‑time compensator – Design a Type II or Type III compensator based on gain/phase margin requirements.

3. Discretize – Convert the continuous compensator into a difference equation (e.g., using the bilinear transform).

4. Implement in code – Execute the difference equation in the DSP interrupt service routine and update the PWM/PFM output.

3.2 Selection of Digital Compensator Type

For a 4kW LLC with a switching frequency of 100 kHz and a crossover frequency in the 10‑20 kHz range, a Type II compensator is sufficient to meet the phase margin requirement (≥45°).

3.3 Code Implementation of a Digital Compensator (C Example)

Below is a difference‑equation implementation of a Type II digital compensator (pseudo‑code):

// Coefficients obtained by discretizing a continuous‑time compensatorfloat b0, b1, b2, a1, a2;// Difference equation: y[n] = b0*x[n] + b1*x[n-1] + b2*x[n-2] - a1*y[n-1] - a2*y[n-2]float x1, x2, y1, y2;// Past statesfloatPID_update(float error){float y = b0*error + b1*x1 + b2*x2 - a1*y1 - a2*y2;
    x2 = x1;
    x1 = error;
    y2 = y1;
    y1 = y;return y;}

• The loop update rate is typically set to 1/2 or 1/4 of the switching frequency (i.e., 25‑50 kHz).

• The computation should be placed in a timer interrupt or PWM interrupt to ensure deterministic timing.

3.4 PFM Frequency Control

An LLC converter uses frequency modulation (PFM). The DSP must map the compensator output to a switching frequency.

• The compensator output (normally normalized to 0‑1) corresponds to a frequency range (e.g., 80‑150 kHz).

• Use the high‑resolution PWM module of the DSP (e.g., TI’s HRPWM) to generate a square wave with adjustable period.

• Pay attention to dead‑time setting (typically 200‑300 ns) to avoid shoot‑through.

IV. Software Protection and Communication Functions

4.1 Protection Features Integration

A DSP can easily implement multiple protection logic without adding extra hardware.

4.2 Communication and Remote Monitoring

The DSP can communicate with external equipment via CAN or UART, enabling digital management of the power supply.

• Real‑time upload – Output voltage, current, frequency, temperature, fault codes, etc.

• Remote control – Adjust output voltage, modify protection thresholds, perform firmware updates.

The Levane Power HLF4K series is equipped with a CAN 2.0B interface as standard, allowing seamless integration into drone flight control systems or ground stations.

V. Practical DSP Feedback Design of the Levane Power HLF4K Series

Taking the HLF4K‑D60‑800 as an example, the key parameters of the digital feedback circuit are:

In a load step test (50% → 100%), the output voltage recovery time is less than 1 ms and the overshoot is less than 3%, satisfying the high dynamic requirements of tethered drone power systems.

VI. Layout and Tuning Recommendations

6.1 PCB Layout Key Points

• Sensing resistors: Place close to the output terminals. Route differential signal lines in parallel, avoiding crossing power loops.

• DSP and ADC: Connect analog ground and digital ground at a single point. Use an independent high‑precision reference for the ADC reference voltage (VREF).

• PWM isolation: Position the digital isolator between the DSP and the gate driver. The isolation bandwidth should be >20 times the switching frequency.

6.2 Tuning Tools and Steps

1. Open‑loop test – First perform an open‑loop frequency sweep to obtain the actual power‑stage Bode plot, verifying the small‑signal model.

2. Closed‑loop tuning – Tune the compensator coefficients off‑line using SIMPLIS, then import them into the DSP.

3. Time‑domain test – Use an electronic load to perform load‑step tests, observe the output voltage waveform, and fine‑tune the crossover frequency.

4. Bode plot measurement – Use a frequency response analyzer (e.g., Venable) to measure the loop gain/phase, confirming that the margins meet the requirements.

VII. Conclusion

The DSP feedback circuit brings the control of a 4kW LLC converter into the digital domain. It overcomes the problems of component tolerance and temperature drift inherent in traditional analog loops, and, through software, endows the power supply with intelligence. The Levane Power HLF4K series, thanks to the powerful processing capability of the C2000 DSP, achieves high efficiency, high reliability, and high flexibility.

With this article, we have completed the full design series for the power stage and control stage of a 4kW LLC converter. Every carefully chosen component and optimization – from the transformer, power switches, rectifier diodes, output capacitors, to the feedback circuit – collectively builds the outstanding reputation of the HLF4K series in the tethered drone power supply market. In the future, we will continue to explore topics such as thermal design and EMC. Stay tuned.

About Levane Power

Levane Power Technology Co., Ltd. focuses on the R&D and manufacturing of DC‑DC power products for new energy applications. It provides customised, high‑efficiency, high‑power‑density, and high‑reliability power solutions for tethered drones, vehicle systems, industrial equipment, and more.

For more product information or technical support, please visit the Levane Power website or contact the technical team.

Series articles review:

• [4kW LLC Transformer Design Deep Dive]

• [4kW LLC Converter 1200V SiC MOSFET Selection Guide]

• [4kW LLC Converter Secondary Rectifier Diode Selection Guide]

• [4kW LLC Converter Output Filter Capacitor Selection Guide]

• This article: 4kW LLC Converter DSP Feedback Circuit Design Guide (Digital Version)