Maintain an efficient and reliable design scheme in the power system

“The capacitance at the input of the DC-DC converter plays an important role in maintaining the stability of the converter and helps filter out electromagnetic interference (EMI) at the input. The large capacitance at the output of the DC-DC converter will bring arduous challenges to the power system. Many downstream loads of DC-DC converters require capacitors to function properly. These loads can be pulsed power amplifiers or other converters that require capacitance at the input. If the capacitance value of the load side exceeds the limit that the DC power system design can handle, the current of the power system may exceed its maximum rating during startup and normal operation.Capacitors can also cause the stability of the power system

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The capacitance at the input of the DC-DC converter plays an important role in maintaining the stability of the converter and helps filter out electromagnetic interference (EMI) at the input. The large capacitance at the output of the DC-DC converter will bring arduous challenges to the power system. Many downstream loads of DC-DC converters require capacitors to function properly. These loads can be pulsed power amplifiers or other converters that require capacitance at the input. If the capacitance value of the load side exceeds the limit that the DC power system design can handle, the current of the power system may exceed its maximum rating during startup and normal operation. Capacitors can also cause stability problems in the power system, leading to erroneous system operation and premature power system failure.

In the case of supplying power to large capacitive loads, implementing some simple techniques in the power supply system can maintain high-efficiency and reliable design. Shortening the voltage rise time applied across the load capacitor at startup can keep the current of the power system within its rated range. During normal operation, controlling the charging current flowing into the capacitor can keep the power system within the rated range, and adjust the control loop of the system The Circuit can maintain the stability of the power system and keep the voltage of the power system within its rated range.

Considerations when starting

When the power system is started, a typical DC-DC converter has a standard rise time, which is determined by the rise time of the internal error amplifier reference. The discharge capacitor placed at the output of the converter will appear as a low impedance load. In the case of this low output impedance, a few switching cycles of the converter may cause a sufficiently high voltage change on the capacitor and force the converter’s output current to exceed its rated value. This capacitor can be precharged through the higher impedance path at the converter output. This high impedance element will limit the charging current into the capacitor until the capacitor is charged to a predefined voltage value. Once the pre-defined voltage value is reached, the high-impedance path can be removed or a low-impedance device (such as a FET) can be used to short-circuit it.

The converter can provide the maximum rated current through this lower impedance path. When the FET short-Circuits this impedance path, it will allow the full-scale voltage of the converter to charge the capacitor. The conduction time of the FET and the voltage difference between the capacitor and the converter voltage determine the charging current required to charge the capacitor to the full-scale voltage. Therefore, setting the predefined voltage value as the FET conduction will not cause the converter to exceed it. The point of rated current is very important. The block diagram shown in Figure 1 can be used to charge the capacitor to a preset minimum voltage. U2 is used to control the FET to short-circuit the resistance Z when necessary, and the U1 circuit and U2 are used together to set the turn-on voltage and load enable.

Figure 1: Block diagram of capacitor pre-charging

At startup, the converter regards the capacitor as the load and the system load after the capacitor. If the system load needs to consume current from the capacitor during the high-impedance pre-charging period, the capacitor may not reach the preset charging voltage. Many downstream loads of the DC-DC converter have an under-voltage lockout function, and they only require a small current in the under-voltage lockout state. If the load does not have an undervoltage lockout function above the preset charging voltage, then an external enable signal should be used. If the load itself is resistive, you can use a series switch to enable the voltage to the load after the capacitor is charged. Figure 2 shows the voltage and current of a system charging a 10mF capacitor.

Figure 2: 12V DC converter charging a 10KuF capacitor

Once the capacitor is charged, the load can start drawing current from the capacitor and the DC-DC converter. Some loads require fast access to current. If this requirement exceeds the bandwidth of the converter, the current will be provided by the capacitor. Once the current is provided by the capacitor, the voltage on the capacitor will drop:

Among them, Vdrop is the voltage drop on the capacitor, I is the required current value, C is the capacitance value, and dt is the duration of drawing current. The converter will recharge the capacitor to its original value. In doing so, the converter output current may exceed its rated value. The voltage difference between the converter and the fully discharged capacitor divided by the resistance between the two voltages determines the desired recharge current. In order to reduce system losses, the resistance between the two voltages is usually very low, so the desired recharging current may be higher than the converter’s maximum value. Since the capacitor voltage is close to the set point voltage of the converter, exceeding the maximum current value of the converter may mean exceeding the maximum power value of the converter.

In order to prevent the converter from exceeding its rated current and rated power during normal operation, the current control block diagram in Figure 3 can be used to control the recharging current after a high di/dt event. This circuit can monitor the current on the shunt resistor and limit the recharge current by actively lowering the converter voltage. This limited voltage difference between the converter and the capacitor will limit the recharging current of the capacitor, thereby ensuring that the converter is within its current and power limits. When the capacitor voltage rises, the converter voltage also rises until it reaches its set value.

The current limiting method shown in Figure 3 can be combined with the precharge method in Figure 1 to achieve a faster startup process. The pre-charging circuit can charge the capacitor to the minimum regulated voltage of the converter, and then the converter charges the capacitor at full speed with the maximum rated current. Controlling the rate of increase of the output voltage can achieve the purpose of controlling the current charged to the capacitor. However, most DC-DC converters have only a narrow control or adjustment range from their nominal set voltage. The typical adjustment interval is ±10%. Some manufacturers can provide a wider adjustment range, and the converter can even be adjusted down to -90% of the nominal set voltage. The smaller the voltage adjustment range, the lower the requirements for the enable circuit, because downstream loads usually have an undervoltage lockout function when they are close to their minimum operating voltage.

Figure 3: Block diagram of external current limit

Stability considerations

Once the converter is kept within its limits during startup and operation, then we must then ensure the stability of the system. The large capacitance at the output of the DC-DC converter may reduce the phase margin of the system and cause ringing. In order to ensure the stable operation of the converter, there must be a minimum value of resistor and capacitor in series for use. Lead or wire resistance, FET and capacitor equivalent series resistance are all effective components of this resistance. The best way to find the minimum value of this resistance is to use a network analyzer and run the system analysis function to determine the margin of phase and gain. If you don’t have a network analyzer, you can also connect a step load in the system to analyze the converter’s voltage and current waveforms to ensure that there is no excessive ringing that represents poor stability.

Once the voltage loop stabilizes, you can check the current control loop in Figure 3 and analyze its impact on system stability. This current control loop is located in the control loop of the DC-DC converter, and its bandwidth should be much smaller than the crossover frequency of the system loop, so the two loops will not interact. In a converter system where the power compensation network is integrated inside the converter, the converter manufacturer can provide enough information to set an appropriate crossover frequency for the current control loop. Some converter manufacturers allow designers to optimize the performance of specific applications by adjusting the power control loop.

Figure 4 shows a converter with an external control loop. This control loop can be optimized to provide peak system performance. In applications where the response time of the power supply system is critical to correct system operation, this external control loop is very important. This is the case for periodic pulse load applications, where the converter must recharge the capacitor before the next power pulse. A network analyzer or step load test should be used to verify the stability of the system. An unstable system may produce voltage excursions that exceed the rated value of the power system components, and eventually cause the power system to fail.