Overview
With the growth of electric vehicles, demand for power battery chargers is increasing. Charger quality affects battery performance and lifetime, and charger intelligence influences user convenience and power system billing and management. Different battery chemistries require different charging strategies. A well-developed charger for one battery type can be adapted to others. This topic is practical and relevant for training in power electronics and charging-system development.
Main Specifications
- Input: single-phase 50Hz +/-10%; RMS voltage variation range 220V +/-20% (176V–264V).
- DC output rated voltage: 50V.
- Input-side power factor correction implemented; target power factor: 0.90.
- Initial charging efficiency greater than 80%.
- Input current distortion less than 4%.
- Charging stages: activation, fast charge, and float charge.
- Temperature monitoring with charging strategy adjustment based on battery and ambient temperature.
- Human-machine interface enabling adjustment of charging parameters.
- Cooling method: forced air cooling.
Filtering Circuit
EMI filter circuit.
C1 and L1 form the first-stage EMI filter. C2, C3, C4 and L2 form the second-stage filter. L1 and L2 are common-mode inductors.
Rectification and Power Factor Correction
Rectifier bridge:
- Diode forward current ID = 3.55 A.
- Required diode reverse voltage V = 373 V.
- Selected bridge: BR601 (35 A / 1000 V) to meet practical operating margins.
Power factor correction:
Topology choice: A boost PFC topology is selected due to low output impedance, simple hardware and control, and mature technology.
Controller selection: UCC28019 supports 100 W–2 kW and can achieve power factor up to 0.95, satisfying the design requirements.
Rectification and PFC circuit diagram.
DC-DC Main Topology
Topology choice:
Under conditions where switching devices see peak currents and voltages, a full-bridge converter provides twice the output power of a half-bridge for the same device stress. For power above 500 W, a full-bridge topology is more appropriate; therefore a full-bridge topology is used.
Power switch selection:
After rectification and filtering, the maximum voltage is about 373 V and maximum primary current about 3.5 A. The chosen MOSFET is FQA24N50 (24 A / 500 V, Rds(on) approx. 0.2 Ω).
Output rectifier diodes:
The required maximum reverse voltage is about 100 V and current about 10 A. Selected diode: MUR3060 (600 V / 30 A).
Full-bridge power stage diagram.
Rectified Output Filter
Rectification and output filtering stage providing DC output smoothing for the battery charging output.
Driver Circuit
PWM signals are isolated via optocouplers and passed through inverters into half-bridge drivers (IR2110). The Q1/Q2 half-bridge driver stage is shown; Q3/Q4 use the same driver arrangement.
Auxiliary Power Supply
System block diagram of the auxiliary supply.
The auxiliary supply provides stable 12 V, 5 V and -12 V rails with high efficiency to power control and gate-drive circuits.
Control and Protection
Control PWM section:
The PWM controller used for the power-stage control is SG3525. The control circuitry implements PWM generation and loop control.
Sampling circuits:
Voltage, current, and temperature sampling circuits provide closed-loop feedback for charger control.
Thermal protection:
The system monitors battery temperature and charger temperature. If the battery overheats, PWM output is disabled and the microcontroller signals an alarm. If the charger overheats, the cooling fan is enabled; if temperature continues to rise, the charger will shut down.
Overcurrent and short-circuit protection:
If the output current exceeds 12 A, the system activates current limiting and issues an alarm. Before charger startup, a short-circuit test is performed; if the measured resistance is below 0.5 Ω, the system reports a fault and prevents startup.
Software Structure
The embedded software organizes control into modular tasks for sampling, control loops, protection, and human-machine interface. Inter-module remote interrupt techniques are used for software-level interference suppression.
Four-Stage Charging Strategy
Charging stages and parameters:
- Activation charge: After the charger starts, the microcontroller measures battery terminal voltage. If the voltage is very low, indicating deep discharge, a low-current activation charge is applied to avoid excessive inrush.
- Constant-current charge: Charge current set to 10 A.
- Constant-voltage charge: Charge voltage set to 59 V.
- Trickle/float charge: When the charge current falls to 0.1 times the constant current (about 1 A), the charger switches to trickle/float mode.
This four-stage strategy ensures initial activation and recovery of deeply discharged batteries, prevents overcharge at the end of charging, and achieves a full charge safely.
EMC and Interference Mitigation
Hardware measures:
- Rectifier diodes use Schottky devices where appropriate to reduce switching losses.
- RCD snubber networks are applied on switching loops to suppress voltage spikes.
- Input EMI filter and optimized transformer design reduce conducted and radiated emissions.
- PCB layout and trace routing are optimized to minimize loop areas and coupling.
Software measures:
Implement modular software design with interrupt and task isolation to reduce susceptibility to disturbances and ensure reliable control.
