Principles and Maintenance of Electric Vehicle Chargers

Common electric vehicle chargers can be broadly categorised into two types based on circuit structure. The first type employs a single-transistor switching power supply driven by the UC3842 to control a field-effect transistor, utilising an LM358 dual operational amplifier to implement a three-stage charging method. 220V AC power is filtered and interference suppressed via the T0 bidirectional filter, rectified by D1 into pulsating DC, then filtered through C11 to produce a stable DC output of approximately 300V. U1 is a TL3842 pulse width modulation integrated circuit. Pin 5 serves as the power supply negative terminal, pin 7 as the positive terminal, and pin 6 outputs pulses directly driving the field effect transistor Q1 (K1358). Pin 3 controls maximum current limiting; adjusting the resistance of R25 (2.5 ohms) modifies the charger's maximum current. Pin 2 provides voltage feedback, enabling adjustment of the charger's output voltage. Pin 4 connects to the external oscillation resistor R1 and oscillation capacitor C1. T1 is the high-frequency pulse transformer, serving three functions: firstly, it steps down the high-voltage pulses to low-voltage pulses; secondly, it isolates the high voltage to prevent electric shock; Thirdly, it supplies operating power to the UC3842. D4 is the high-frequency rectifier diode (16A 60V), C10 is the low-voltage filter capacitor, D5 is the 12V zener diode, and U3 (TL431) is the precision reference voltage source. Together with U2 (optocoupler 4N35), it enables automatic regulation of the charger's output voltage. Adjusting W2 (trimming resistor) allows fine-tuning of the charger voltage. D10 is the power indicator LED. D6 is the charging indicator LED. R27 is the current sensing resistor (0.1Ω, 5W). Altering the resistance value of W1 adjusts the charger's float charge transition threshold current (200–300mA).

Upon power-up, approximately 300V is present across C11. One branch of this voltage is applied to Q1 via T1. The second branch reaches pin 7 of U1 via R5, C8, and C3, forcing U1 to activate. Pin 6 of U1 outputs square-wave pulses, activating Q1. Current flows through R25 to ground. Simultaneously, the secondary winding of T1 generates an induced voltage, which, via D3 and R12, provides a reliable power supply to U1. The voltage from T1's primary winding is rectified and filtered through D4 and C10 to produce a stable voltage. One branch of this voltage, via D7 (which prevents reverse current flow from the battery back to the charger), charges the battery. The second branch supplies 12V to the LM358 (dual operational amplifier, pin 1 being power ground, pin 8 being power positive) and its peripheral circuitry via R14, D5, and C9. D9 provides the reference voltage for the LM358, which is divided by R26 and R4 to reach pins 2 and 5 of the LM358. During normal charging, a voltage of approximately 0.15–0.18V appears across the upper terminal of R27. This voltage is applied to pin 3 of the LM358 via R17, causing a high voltage to be output from pin 1. One branch of this voltage passes through R18, forcing Q2 to conduct and illuminating D6 (red LED). while another branch injects into pins 6 and 7 of the LM358, outputting a low voltage that forces Q3 to turn off. D10 (green LED) extinguishes, and the charger enters the constant-current charging phase. When the battery voltage rises to approximately 44.2V, the charger transitions to the constant-voltage charging phase, maintaining an output voltage around 44.2V while the charging current gradually decreases. When the charging current reduces to 200mA–300mA, the voltage across R27 decreases. The voltage at pin 3 of the LM358 falls below that at pin 2, causing pin 1 to output a low voltage. Q2 turns off and D6 extinguishes. Simultaneously, pin 7 outputs a high voltage. This voltage activates Q3 via one path, causing D10 to illuminate. Another path travels via D8 and W1 to the feedback circuit, causing the voltage to decrease. The charger then enters the trickle charging phase. Charging concludes after 1–2 hours.

Common faults in chargers fall into three main categories: 1: High-voltage faults 2: Low-voltage faults 3: Faults affecting both high and low voltages. The primary symptom of a high-voltage fault is the indicator light failing to illuminate. Characteristic indicators include: - Blown fuse - Breakdown of rectifier diode D1 - Bulging or bursting of capacitor C11 - Breakdown of transistor Q1 - Open circuit in resistor R25 Short circuit between pin 7 of U1 and ground. Open circuit in R5, resulting in no start-up voltage for U1. Replacing these components should resolve the issue. If pin 7 of U1 shows over 11V and pin 8 shows 5V, U1 is essentially functional. Focus testing should be directed at checking for cold solder joints on the pins of Q1 and T1. Should Q1 repeatedly break down without overheating, this typically indicates failure of D2 or C4. If Q1 breaks down whilst overheating, this generally signifies leakage or short-circuit in the low-voltage section, excessive current, or abnormal pulse waveform at pin 6 of the UC3842. This causes significantly increased switching losses and heat generation in Q1, leading to its overheating and burnout. Other manifestations of high-voltage faults include indicator light flickering, low and unstable output voltage. These are typically caused by poor soldering at T1's pins, open circuits in D3 or R12, or lack of operating power to the TL3842 and its peripheral circuitry. A rare high-voltage fault manifests as excessively high output voltage exceeding 120V. This is usually caused by U2 failure, an open circuit in R13, or breakdown of U3, which pulls down the voltage at pin 2 of U1 and causes pin 6 to output excessively wide pulses. Prolonged operation under these conditions must be avoided, as it will severely damage the low-voltage circuitry.

Most low-voltage faults stem from reverse polarity connection between charger and battery terminals, causing R27 to burn out and the LM358 to break down. Symptoms include a continuously lit red indicator, unlit green indicator, low output voltage, or output voltage approaching 0V. Replacement of the aforementioned components will resolve the issue. Additionally, output voltage drift due to W2 oscillation may occur. If the output voltage is excessively high, the battery may overcharge, leading to severe dehydration, overheating, and ultimately thermal runaway causing an explosion. Conversely, an excessively low output voltage will result in undercharging.

When faults occur in both high and low voltage circuits, conduct a comprehensive inspection of all diodes, transistors, optocouplers (4N35), field-effect transistors, electrolytic capacitors, integrated circuits, and resistors R25, R5, R12, R27—particularly D4 (16A 60V fast recovery diode) and C10 (63V 470μF)—prior to powering up. Avoid blindly applying power, which may further expand the fault scope. Some chargers incorporate reverse polarity and short-circuit protection at the output stage. This essentially adds a relay to the output circuit; during reverse polarity or short-circuit conditions, the relay fails to operate, preventing voltage output from the charger.

Other chargers also feature reverse polarity and short-circuit protection, though their principle differs from the aforementioned design. Their low-voltage circuit draws its start-up voltage from the battery being charged and incorporates a diode (reverse polarity protection). Once the power supply is properly activated, the charger then supplies the low-voltage operating power. The control chip in such chargers is typically based on the TL494, driving two 13007 high-voltage transistors. Combined with the LM324 (four operational amplifiers), this achieves three-stage charging.

The 220V AC is rectified via D1-D4 and filtered by C5 to yield approximately 300V DC. This voltage charges C4, forming the starting current through TF1's high-voltage winding, TF2's primary winding, and V2. The feedback winding of TF2 generates an induced voltage, causing V1 and V2 to conduct alternately. Consequently, a voltage is produced in the low-voltage supply winding of TF1. This voltage is rectified via D9 and D10, filtered by C8, and supplies power to components such as TL494, LM324, V3, and V4. At this stage, the output voltage remains relatively low. Upon activation, the TL494 alternately outputs pulses from pins 8 and 11, driving V3 and V4. These pulses, via the TF2 feedback winding, excite V1 and V2. This transitions V1 and V2 from self-oscillating to controlled operation. The output winding voltage of TF2 rises. This voltage is fed back to pin 1 of the TL494 (voltage feedback) via voltage division across R29, R26, and R27, stabilising the output voltage at 41.2V. R30 serves as the current sense resistor, generating a voltage drop during charging. This voltage is fed back via R11 and R12 to pin 15 of the TL494 (current feedback), maintaining the charging current at approximately 1.8A. Additionally, the charging current creates a voltage drop across D20, which is conducted through R42 to pin 3 of the LM324. This causes pin 2 to output a high voltage, illuminating the charging indicator, while pin 7 outputs a low voltage, extinguishing the float charge indicator. The charger enters the constant-current charging phase. Moreover, the low voltage at pin 7 pulls down the anode voltage of D19. This reduces the voltage at pin 1 of the TL494, causing the charger's maximum output voltage to reach 44.8V. When the battery voltage rises to 44.8V, the constant-voltage phase commences.

When the charging current drops to 0.3A–0.4A, the voltage at pin 3 of the LM324 decreases. Pin 1 outputs a low voltage, extinguishing the charging indicator. Simultaneously, pin 7 outputs a high voltage, illuminating the float charge indicator. Moreover, the high voltage at pin 7 raises the anode voltage of D19. This increases the voltage at pin 1 of the TL494, causing the charger's output voltage to decrease to 41.2V. The charger enters float charge mode.

Example:

Charger. Upon connecting the power supply, the charger shows no response whatsoever. However, the storage capacitor retains charge. If not promptly discharged here, it may deliver a startling jolt, causing considerable discomfort.

First ascertain whether the 13007 is functional. Measure the midpoint voltage between the two transistors; if it reads 150V, the issue lies between the 68μF/400V capacitor and the main transformer circuit. If not 150V, one of the two 240K start-up resistors is faulty. The latter scenario is more common. For 3842 circuits, the start-up resistor typically becomes an infinite impedance; the two 2.2 ohm resistors should also be checked.