Using "soft start circuit" to eliminate switching power supply surge current

Central topics:

• Causes of surge current in switching power supplies

• Electrical working principle of soft start circuit

• Soft start circuit considerations

Solution:

• Removes loads at power-up instant while limiting useful current

• All transistors are turned off when power is first applied

Among the various power supplies commonly used in the past and present, switching power supplies are very popular and can generally meet any design requirements. This type of power supply is very economical, but there are also some problems in the design. That is, many switching power supplies (especially high-power switching power supplies) have an inherent disadvantage: they draw a large current at the moment of power-on. This surge current may reach 10 to 100 times the static operating current of the power supply.

This may cause at least two problems. First, if the DC power supply cannot supply enough starting current, the switching power supply may enter a locked state and fail to start. Second, this surge current may cause the input power supply voltage to drop, which is enough to cause other power equipment using the same input power supply to lose power instantly.

The traditional input inrush current limiting method is to connect a negative temperature coefficient thermistor (NTC) in series. However, this simple method has many disadvantages: the current limiting effect of the NTC resistor is greatly affected by the ambient temperature, the current limiting effect can only be partially achieved during a short input main grid interruption (about a few hundred milliseconds), and the power loss of the NTC resistor reduces the conversion efficiency of the switching power supply. In fact, the two problems mentioned above can be solved by a "soft start circuit", which is introduced in detail below.

Causes of Surge Current in Switching Power Supply

Before discussing the "soft start circuit", let's first discuss how surge current is generated. Modern drive systems, inverters, and switching power supplies generally convert electrical energy through pulse modulation technology (PWM), and the core component is the DC/DC converter. In the switching power supply shown in Figure 1, the input voltage is first filtered through interference, then converted into DC through a bridge rectifier, and then the waveform is smoothed through a large electrolytic capacitor before entering the real DC/DC converter. The input surge current is generated when the electrolytic capacitor is initially charged. Its size depends on the amplitude of the input voltage at startup and the total resistance of the loop formed by the bridge rectifier and the electrolytic capacitor. If it is started at the peak point of the AC input voltage, a peak input surge current will appear.

In addition, input surge current will also appear when the transformer power supply starts. However, the reason for this input surge current is different. When the transformer power supply starts at the zero-crossing point of the sinusoidal input voltage, the magnetization of the transformer core is forced into an unbalanced state in the first few cycles. As a result, the core saturates in each half cycle. At this time, the excitation current can only be limited by the weak leakage inductance parasitic resistance, resulting in a large input surge current. Transformer power supplies usually have special input surge current limiters to ensure that they start at the peak of the sinusoidal input voltage to prevent high input surge currents. If this input surge current limiter is also used in a switching power supply, as mentioned above, the consequences are exactly the opposite. Not only will it not play a current limiting role, but it will also cause a peak input surge current. Therefore, today we will only discuss the generation and elimination of surge currents in switching power supplies, and transformer power supplies are not within the scope of discussion.

Electrical working principle of soft start circuit

If the "soft start circuit" we designed today is used to eliminate the inrush current when the switching power supply starts, the shortcomings of the above-mentioned traditional inrush current limiting method can be well avoided. Controlling the startup of the switching power supply through "soft start" to eliminate the inrush current includes two design principles: removing the load at the moment of power-on and limiting the useful current at the same time. If the load is not driven, the current of the switching power supply is generally very small when it starts. In many cases, the actual startup current may be smaller than the steady-state operating current maintained by this method.

The following uses a switching power supply circuit from -48V to +5V to discuss the "soft start" technology. The switching power supply used is a voltage regulator containing LT1172HVCT, a buck-boost converter from negative to positive compensation. In fact, any switching power supply from -48V to +5V can work. Among them, the soft start circuit and the switching power supply circuit are independent of each other, and the electrical principle is shown in Figure 2.

The working principle of the circuit is very simple. When the power is turned on, all transistors are turned off, C1 is in a discharge state, the load is disconnected, and the input current is shunted by the current limiting resistor R4. When the switching power supply starts, its output voltage begins to rise. When the output voltage reaches 4.5V (3.9V across D1 plus Veb of Q3 = 0.6V), Q3 turns on and charges C1. When the voltage VC across C1 reaches the threshold voltage of Q1 (usually 3V), Q1 turns on.

VC continues to rise, Q1 is fully turned on, providing a low impedance path for the input current and effectively bypassing the current limiting resistor R4. When VC reaches 7.4V (6.8V across D2 plus Q4's Vbe = 0.6V), Q4 is turned on and provides bias to Q2, which also turns on Q2. This connects the load to the power supply through a low impedance. At this point, the power supply has been safely started and the soft start circuit has completed its function. The on-time of Q1 and Q2 can be calculated using the following formula:

When VC is equal to 3V, Q1 is turned on, that is, it is turned on about 150ms after the output of the power supply reaches 4.5V; when VC is equal to 7.4V, Q2 is turned on, that is, it is turned on 330ms after Q1 is turned on. Such a long time is enough to ensure the required stabilization time of the power supply and make Q1 and Q2 turn on slowly. Because the starting current must be kept at a minimum value, the slow turn-on of the FET (field effect transistor) is crucial. If the FET switches too quickly, it may generate a large surge current and lose the effectiveness of the soft start circuit.

Precautions

(1) The addition of a soft start circuit comes at a price. Overall, this circuit can be considered as part of the power supply. It consumes power and reduces the efficiency of the power supply. Most of the power loss is caused by the non-zero on-resistance of the output transfer field effect transistor Q2. The on-resistance of this IRFD9210 is 0.6Ω. At an output current of 500mA, Q2 will consume 300mW of power. If such a large loss is not allowed, a FET with a smaller on-resistance can be used (but it is often very expensive).

(2) Because the switching power supply voltage is sensed from the input of the field effect transistor Q2, the resistance across Q2 also affects the stability of the load voltage. As long as the load current is relatively constant, this problem is not serious. If the output voltage changes greatly, you can choose a FET with low on-resistance to improve it, or you can add a voltage sensing circuit to the output of Q2 after the soft start circuit is completed.

The above detailed discussion is how the "soft start circuit" eliminates the surge current of the switching power supply. After multisim software simulation and laboratory practice, it is proved that the control ability of the soft start circuit is very strong. Recently, we have jointly designed a "SF-DC75~100W module power supply" with "Beijing Newport Power Technology Co., Ltd.". This power supply partly utilizes the above design principle. Through market verification, this circuit can indeed eliminate the surge current when the high-power switching power supply is started, and greatly improve the output characteristics of the module power supply. Therefore, it can be predicted that this circuit has good market promotion value. In fact, although the above discussion is limited to "-48V~+5V" switching power supply, it can also be modified into a circuit suitable for various switching power supplies.