Short-circuit Failure Analysis for the Relay Contact at the Input of Switching Power Supply
CONTENTS:
Abstract:
An important performance index of switching power supply is input inrush current, which is usually designed as small as possible. The conventional design is to connect a thermistor (NTC) in series on the live wire at the input of the switching power supply. While for switching power supply with a higher power, a relay is connected in parallel to the thermistor (NTC) at the same time to reduce device loss and improve reliability when the product works normally.
This paper focuses on analyzing the causes of contact short-circuit failure after incorporating the relay. In terms of principle, experimental test, verification, and relay materials, we analyze the problems of the relay in the process of circuit design and applications in detail, so as to provide a reference design of the relay in switching power supply.
Keywords: switching power supply, relay, inrush current
Failure and its Causes
I. Failure phenomenon 1
When we applied the PFC circuit in Figure 1 to the product and tested it, we found that the inrush current exceeded the standard (design objective ≤25A) and reached 70A. If the Inrush current Imax of this case is calculated according to the theory, the resistance value of thermistor RT1 in this scheme is 10Ω, and the input voltage is 90VAC.
When the phase is 90° or 270°, the maximum input peak voltage is
, and the maximum input peak current (input inrush current) is
. The test result is completely different from the theoretical calculation.
Figure 1. PFC Circuit
II. Analysis of Failure phenomenon
According to Figure 1, the devices that affect the input inrush current are mainly thermistor RT1 and relay K1, which have the following four combinations:
① When the thermistor is open-circuit and the relay is suspended, the input is disconnected and the circuit is not turned on.
② When the thermistor is open-circuit and the relay is sucked, the input current is directly sent to the back-end circuit through the relay. The thermistor does not work in the circuit, and the input inrush current is large.
③ When the thermistor is working normally and the relay is suspended, the input current is supplied to the back-end circuit through the thermistor, and the input inrush current is suppressed and reduced.
④ When the thermistor is working normally and the relay is sucked, the input current is mainly supplied to the back-end circuit through the relay. The thermistor does not work in the circuit, and the input inrush current is large.
Conclusion: Tested the thermistor and relay, and we found that the thermistor resistance is normal, and the normally open contact of the relay is sucked without a power supply, that is, the relay is abnormal device. After replacing a new relay, the measured inrush current is only 7.4A. In the previous test, the inrush current exceeding the standard belongs to the assumption ④.
III. The operating principle of circuit in Figure 1
The operating principle of circuit in Figure 1 is that relay K1 is connected in parallel to thermistor RT1, and powered by the auxiliary winding of PFC inductance L2 after linear voltage stabilization. When the switching power supply is powered on, as relay K1 has no power supply voltage at this time, relay K1 is in an open-circuit state, and the input current charges the large electrolytic capacitor C8 through thermistor RT1, thus limiting the input inrush current of startup.
When the power tube Q1 receives the driving signal, the auxiliary winding voltage of PFC inductance L2 is established, that is, the power supply voltage of relay K1 is established. When the power supply voltage reaches about 9V, the relay starts to work, the relay contact is closed and the thermistor RT1 short-circuits, so as to reduce the input line impedance and loss and improve the efficiency of the product.
Causes for short circuit failure of relay contacts
I. Assumption
When the relay is not powered on and the normally open contact has been closed, that is, the relay contact is short-circuited. There are generally three possibilities accounting for the failure. The following possible causes are analyzed and investigated one by one:
① The operation frequency of the relay is too high, and the times have exceeded the switching times that the relay can bear.
② The surrounding temperature of the relay is too high.
③ The surge current flowing through the relay is too large.
II. Verification
Through the analysis of the operating principle of the circuit in Figure 1 and the monitoring of the voltage at both ends of the contact of relay K1, we found that K1 only operates during power on, and the contact will not switch after normal operation.
Therefore, the switching times of relay K1 are only related to the switching times inputted manually. Referring to the relay specification, the service times of the relay are 1 * 104 times. As the product is still in the commissioning stage, it is impossible to reach 1 * 104 times, which means the failure is not caused by being used more than it should be.
Figure 2. Steady-state current waveform of relay contact
[The yellow waveform is the input voltage and blue one is the relay contact current.]
Through measurement, as shown in Figure 2, the contact current of the relay is about 3A and the surrounding temperature of the relay is 83 ℃. Referring to the relay specification applied in this case, it is indicated that the surrounding temperature resistance parameter is 10A / 85 ℃, which can be used at 105 ℃ when the input current is 7A. Compared with the actual environment and current, it can be ruled out that the failure is caused by the high temperature.
Figure 3. Conduction waveform of relay contact
[The yellow waveform is the input voltage and blue one is the relay contact current.]
III. Conclusion
The load of relay K1 is inductive load (L1, L2) and capacitive load (C1, C2, C8). The measured contact current of relay K1 is shown in Figure 3. From the figure, it can be found that the relay K1 has peak contact current within a period of time after conduction, with a maximum peak of 39.4A. The maximum withstand current of the relay is 10A, and the surge current (39.4A) generated during product commissioning will damage the contact and lead to suspension.
Causes of Surge Current in Relay
I. Assumption
The troubleshooting shows that the failure reason for the relay contact short-circuit is that the surge current flowing through the relay is too large. In the circuit in Figure 1, what is the reason for the surge current during relay attraction? Monitor and analyze the following devices that may cause surge current:
① Whether PFC inductance L2 is saturated?
② Whether L1 differential mode inductance is saturated?
③ Whether π type filter capacitor C1 is too large?
④ Whether the PFC limit clamp current is too large?
II. Verification
The startup current of PFC inductor L2 is shown in Figure 4. The saturation current of PFC is 14A shown in Figure 5. The PFC inductor current is chopped at 13.1A, that is, the PFC current is limited and the Ipeak is 13.1A. The PFC current waveform seems good, and B is less than 0.32 according to the design objective. When the testing current is 13A, the inductance is 180uH (L2 nominal inductance is 190uH), therefore the PFC inductor is unsaturated.
Figure 4. The startup current of PFC inductor L2
Figure 5. PFC inductor is unsaturated
Figure 6. Saturated current
The parameter of the differential mode (DM) inductor L1 is 200uH/48Ts/0.7mm. The measured saturation current is shown in Figure 6. When 13.1A current (the Ipeak when PFC is started) is applied, the inductance is only 12.5uH, the inductance decreases sharply showing that saturation has occurred. At this time, the inductor L1 of the π-filter can not effectively filter the current flowing through relay K1 during the startup of PFC.
After replacing the DM inductor L1 with a DM inductor with a larger saturation current (saturation current is about 16A/200uH), test the contact current and found that the turn-on transient current is 8A. The maximum peak current is 17.4A at conducting state, it is significantly reduced. Contact current before and after replacement please refer to Figure 7 and Figure 8.
Figure 7. Contact current before replacement
Figure 8. Contact current after replacement
[The green waveform is C8 capacitor voltage and the red one is the relay contact current.]
C1 is the first capacitor of π-filter circuit. The input voltage directly charges C1, which will produce a distorted pulse charging current. The larger the capacitance, the larger the distorted current pulse, resulting in a larger peak of relay contact current. Besides the replacement of the DM inductor L1, we replaced a smaller capacitor with a capacitance of 474 / 450V from 683 / 450V, tested the relay contact current, and found that the maximum relay contact current is 8.6A, the current spike is further reduced significantly (previously is 17.4A), as shown in Figure 9.
Figure 9. Current and voltage waveform during PFC boost
[The green waveform is C8 capacitor voltage and the red one is the relay contact current.]
Starting process of PFC control IC: during the boosting process of large electrolytic capacitor C8, the duty cycle of PFC control IC drive output will rise from 0 to the maximum Ton, as shown in Figure 10. The PFC current gradually reaches the PFC current sampling limit and is clamped, as shown in Figure 4. The clamping current of PFC startup is related to the sampling resistance of PFC current.
In the actual applications, the sampling resistance of PFC current is 22mΩ, and the clamping current of PFC is about 13.1A. When resistance reaches 40mΩ, the clamping current decreases, and the inrush current peak decreases.
At the same time, it can also increase the inductance of L1 during startup and increase the effect of PFC π filter, as shown in Figure 11, the maximum contact peak current is 9.6A. The PFC current sampling resistance is related to the overcurrent capacity of the product. Generally, it is not recommended to change this resistor after the overcurrent point is designed.
Figure 10. The startup of PFC
Figure 11. Waveform of relay contact current
III. Conclusion
In conclusion, the large inrush current generated after the relay at the input end is closed can be summarized as follows: the current sampling resistance of PFC is small, that is, the overcurrent point is large, and the input current reaches the clamp point when the PFC starting work (boosting). If the DM inductor of π filter is saturated, it will lose the suppression of current. The larger the capacitance of filter capacitor C1, the larger the distorted current pulse.
Design reference of a relay in switching power supply products
① π-filter circuit at the input: selecting the DM inductor with larger saturation current and reducing the capacitance of the first capacitor in π-filter at the same time.
② Increase the PFC current sampling resistance and reduce the PFC clamping current (this should be balanced with the overcurrent capacity required by the product).
In addition to optimizing the parameters from the circuit, the selection of the relay is also critical. The following will take HONGfa relay HF46F-G series as an example to introduce the difference of its contact material in different applications.
The relay contact material given in the specification is divided into two materials: AgSnO2 and AgNi, that is, the contact material of HF46F-G/XXT (with T) is AgSnO2, while the contact material of HF46F-G/XX (without T) is Agni. The specifications of this series also distinguish the application of different material contacts, as follows:
① AgSnO2 is often used in applications that will produce surge current, such as capacitive load, inductive load, motor load, etc.
② Agni is often used in the context of resistive load and current stability.
For the relay application used at the input of switching power supply, the actual load will generally have devices such as inductance and capacitance that cause surge current, so the relay with AgSnO2 as contact material should be used.
Conclusions
The failures of relays generally include the following: redundant objects inside the relay, dirt on the contact material, improper process structure, contact ablation, adhesion, silver ion migration, and reed displacement caused by external application.
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