A crucial performance indicator for switching power supplies is input inrush current, which is typically designed to be as low as possible. A conventional design involves connecting a thermistor (NTC) in series on the live wire of the power supply input. However, for higher-power switching power supplies, a relay is often connected in parallel with the thermistor (NTC) to reduce component losses and improve reliability during stable operation. This article focuses on analyzing the causes of contact short-circuit failures after incorporating a relay. Through principles, experimental testing, verification, and discussions of relay materials, it provides a detailed analysis of the problems encountered in the application of relays in circuit design, offering a reference for the design of relays in switching power supply products.
Keywords: switching power supply, relay, inrush current
Failure phenomena and sources
In actual engineering design, when the PFC circuit in Figure 1 was implemented in the product, testing revealed that the inrush current exceeded the standard (design target ≤25A), reaching 70A. In this design, the thermistor RT1 has a resistance of 10Ω. Theoretically, based on an input voltage of 90VAC, i.e. at a phase angle of 90° or 270°, the maximum input peak voltage is 90*√2≈127V, and the maximum input peak current (inrush current) is Imax=127/10=12.7A. The test results deviate completely from the theoretical calculations.
Figure 1 PFC circuit
Based on the analysis of Figure 1, the main components affecting the input inrush current are the thermistor RT1 and the relay K1, with the following four combinations: ① Thermistor open circuit and relay not engaged; in this case, the input is open, and the product should have no input. ② Thermistor open circuit and relay engaged; the input current goes directly to the downstream circuit through the relay, the thermistor is ineffective in the circuit, and the input inrush current is large. ③ Thermistor normal circuit and relay not engaged; the input current goes to the downstream circuit through the thermistor, and the input inrush current is suppressed and reduced. ④ Thermistor normal circuit and relay engaged; the input current mainly goes to the downstream circuit through the relay, the thermistor is ineffective in the circuit, and the input inrush current is large. Testing of the thermistor and relay revealed that the thermistor resistance was normal, and the normally open contact of the relay was engaged even without power, indicating that the relay was faulty. After replacing the relay, the measured inrush current was only 7.4A. The previous product test showed that the excessive inrush current was due to the fourth scenario, where the input current mainly went to the downstream circuit through the relay, the thermistor was ineffective in the circuit, resulting in a large input inrush current.
The working principle of circuit diagram 1 is as follows: Relay K1 is connected in parallel across the input thermistor RT1, and is powered by the auxiliary winding of PFC inductor L2 after linear regulation. When the switching power supply is powered on, relay K1 is in an open circuit state because there is no supply voltage at this time. The input current charges the large electrolytic capacitor C8 through the thermistor RT1, thereby limiting the input inrush current during startup. When the power transistor Q1 receives the drive signal, the voltage of the auxiliary winding of PFC inductor L2 is established, that is, the supply voltage of relay K1 is established. When the supply voltage reaches about 9V, the relay starts to work, and the contacts close to short-circuit the thermistor RT1, reducing the input line impedance during product operation, reducing losses, and improving product efficiency.
Causes of relay contact short circuit failure
When the relay is not powered but the normally open contact is already engaged, indicating a short circuit failure of the relay contacts, there are generally three possibilities. The following is an analysis and troubleshooting of each of these three possibilities:
① The relay operates too frequently, exceeding its rated switching frequency;
② The ambient temperature of the relay is too high;
③ The relay is experiencing excessive surge current.
Analysis of the circuit's operating principle in Figure 1 and actual monitoring of the voltage across the contacts of relay K1 show that relay K1 only operates during power-on; after normal operation, the contacts will not switch again. Therefore, the number of times relay K1 switches is only related to the number of manually input switches. Consulting the relay's specifications reveals that its lifespan is 1*10⁴ cycles. Since the product is still in the debugging phase, it's impossible for it to reach 1*10⁴ cycles. Therefore, the issue is not due to exceeding its lifespan.
Figure 2. Steady-state current waveform of relay contact
[Yellow represents input voltage, blue represents relay contact current]
Actual measurements, as shown in Figure 2, indicate that the relay's contact current is approximately 3A during operation, and the ambient temperature is 83℃. The relay's specifications state an ambient temperature resistance of 10A/85℃, and it can be used at 105℃ with a current of 7A. Comparing these measurements with the actual operating environment and current, the possibility of excessively high ambient temperature being the cause can be ruled out.
Figure 3. Relay contact conduction waveform
[Yellow represents input voltage, blue represents relay contact current]
The load downstream of relay K1 consists of inductive (L1, L2) and capacitive (C1, C2, C8) loads. The measured contact current of relay K1 is shown in Figure 3. The figure shows that a current spike occurred at the relay K1 contact for a period after it was turned on, with a maximum spike Imax = 39.4A. The relay's specifications state a maximum current withstand of 10A. However, the surge current impact (39.4A) generated during repeated power-on adjustments during product debugging can damage the contacts, leading to adhesive failure.
Causes of surge current during relay activation
Through investigation, it was determined that the relay contact short-circuit failure was caused by excessive surge current flowing through the relay. Therefore, in the circuit shown in Figure 1, what caused the surge current during relay activation? The following components that may have caused the surge current were monitored and analyzed:
① Is the PFC inductor L2 saturated?
② Is the L1 differential mode inductor saturated?
③ Is the π-type filter capacitor C1 too large?
④ Is the PFC limiting clamping current too high?
The start-up current of PFC inductor L2 is monitored as shown in Figure 4. At this time, the PFC inductor current is clipped, that is, the PFC current is limited and clamped at 13.1A. The PFC current waveform is good, and B<0.32. When a current of 13A is applied, the inductance is 180uH (the nominal inductance of L2 is 190uH). The PFC inductor is not saturated, as shown in Figure 5.
Figure 4 Monitoring the start-up current of PFC inductor L2
Figure 5. PFC inductor is not saturated.
Figure 6 Saturation current
The L1 differential mode inductor has parameters of 200uH/48Ts/0.7mm. Its measured saturation current is shown in Figure 6. When a current of 13.1A (the clamping current during PFC startup) is applied, the inductance is only 12.5uH, indicating a sharp drop in inductance and saturation. At this point, the π-type filter inductor L1 can no longer effectively filter the current flowing through relay K1 during PFC startup. Replacing the L1 differential mode inductor with a larger saturation current (approximately 16A/200uH) and testing its contact current revealed a transient current of 8A upon conduction, with a maximum current spike of 17.4A after conduction. The contact current spike was significantly reduced. Figures 7 and 8 show the results before and after replacement.
Figure 7. Current of the contact before replacement
Figure 8. Contact current after replacement
[Green represents the voltage across capacitor C8, and red represents the relay contact current.]
C1 is the first capacitor in the π-type filter circuit. The input voltage directly charges C1, generating a distorted pulse charging current. The larger the capacitor, the larger the distorted current pulse, resulting in a larger peak value of the relay contact current. After replacing the differential-mode inductor L1, the capacitance of C1 was reduced from 474/450V to 683/450V. Testing the relay contact current revealed a maximum current of 8.6A, with a significant reduction in the current spike (previously 17.4A), as shown in Figure 9.
Figure 9 Current and voltage waveforms during PFC boost.
[Green represents the voltage across capacitor C8, and red represents the relay contact current.]
PFC control IC startup process: During the voltage boosting process of the large electrolytic capacitor C8, the duty cycle of the PFC control IC drive output will increase from 0 to the maximum Ton max, as shown in Figure 10. The PFC current gradually reaches the PFC current sampling limit and is thus clamped, as shown in Figure 4. The PFC startup clamping current is related to the PFC current sampling resistor. In actual engineering design, the PFC current sampling resistor R = 22mΩ, and the PFC clamping current is about 13.1A. Increasing R = 40mΩ reduces the clamping current and the inrush current spike, and also increases the inductance of L1 during startup, increasing the PFC π-type filtering effect, as shown in Figure 11, with a maximum contact current spike of 9.6A. The PFC current sampling resistor is directly related to the product's overcurrent capability. Generally, once the overcurrent point is designed, it is not recommended to change this resistor.
Figure 10 PFC Startup
Figure 11 Relay contact current waveform
In summary, the large inrush current after the input relay is closed can be attributed to the following: a small PFC current sampling resistor means a large overcurrent point; when the PFC starts working (boost), the input current reaches the clamping point with a large current; if the differential-mode inductor of the π-type filter becomes saturated, it will lose its current suppression effect; the larger the value of the filter capacitor C1, the larger the distorted current pulse.
Design reference for relays in switching power supply products
① Input π-type filter circuit: Select a differential-mode inductor with a larger saturation current, and at the same time reduce the capacitance of the first capacitor of the π-type filter.
② Increase the PFC current sampling resistor and decrease the PFC clamping current (this needs to be balanced with the product's required overcurrent capacity).
Besides optimizing circuit parameters, relay selection is also crucial. Here, we introduce the differences in application due to different relay contact materials. For example, the Hongfa HF46F-G series relays have two contact materials listed in their specifications: AgSnO2 and AgNi. Specifically, HF46F-G/XXT (with T) uses AgSnO2 contact material; HF46F-G/XX (without T) uses AgNi contact material. The specifications for this series also differentiate the applications of the different contact materials, as follows:
① AgSnO2 is commonly used in applications where inrush currents are generated, such as capacitive loads, inductive loads, and motor loads.
② AgNi is often used in resistive loads and applications where the current is stable.
For relay applications used at the input of switching power supplies, the actual load at the back end usually has devices such as inductors and capacitors that cause surge current. Therefore, when selecting relays, relays with AgSnO2 as the contact material should be used.
Relay failures generally fall into the following categories: foreign matter inside the relay, dirt on the contact surface, improper manufacturing process, contact erosion, adhesion, silver ion migration, and reed displacement caused by external applications.
Most of these failure modes are caused by improper control of the relay manufacturing process. Therefore, for relay manufacturers, improving the production environment and perfecting quality control and inspection systems will play a crucial role in preventing frequent relay failures. Furthermore, users must first select the appropriate type of relay based on their actual usage requirements, and then carefully determine the required functional and physical characteristics (including environmental adaptability requirements, input and output parameters, time parameters, contact life, size, weight, installation dimensions, installation method, and sealing performance). Selecting a suitable relay in this way is also of great importance in avoiding failures during use.
In actual operation, a switching power supply can continue to function normally even if the relay contacts fail due to a short circuit, making it difficult to detect during use. However, once a relay contact fails due to a short circuit, the large input inrush current will affect the reliability of the product and may even trigger an abnormal alarm in the front-end power supply system. To prevent this, it is crucial to carefully select circuit parameters and choose a relay model that matches the circuit's characteristics during the initial design phase.
Note:
Thermistor (NTC): A thermistor is a sensor resistor whose resistance changes with temperature. (NTC thermistor, or Negative Temperature Coefficient thermistor, has a lower resistance as the temperature increases).
PFC stands for "Power Factor Correction". The power factor refers to the relationship between effective power and total power consumption (apparent power), which is the ratio of effective power to total power consumption (apparent power).
