Discussion on the implementation method and common problems of current detection circuit

1 Introduction

This article mainly discusses the problems of current transformer saturation and secondary current droop that are commonly encountered in current detection, and introduces the implementation method of the current detection circuit.

2 Implementation of current detection circuit

There are two main methods for implementing current detection circuits: resistive sensing and current sense transformer detection. In the control circuit of the current loop, the current amplifier usually selects a larger gain, which has the advantage of selecting a smaller resistor to obtain sufficient detection voltage, and the smaller the detection resistor, the smaller the loss. However, in actual circuit design, especially when designing high-power and high-current circuits, the use of resistor detection is not ideal, because the detection resistor has a large loss, up to several watts or even more than ten watts; and it is difficult to find a resistor as small as a few hundred milliohms or tens of milliohms.

There are two types of resistance detection, as shown in Figure 1 and Figure 2.

When using Figure 1 to directly detect the current of the switch tube, a small RC filter circuit must be connected in parallel next to the detection resistor RS, as shown in Figure 3. Because when the switch tube is turned off, the collector capacitor discharges, generating a transient current spike on the current detection resistor. The pulse width and amplitude of this spike are often enough to lock the current amplifier, thereby causing the PWM circuit to fail. In fact, current transformer detection is practical in high-power circuits, as shown in Figure 4.

In order to completely magnetically reset the current transformer, it is necessary to provide the core with equal and opposite volt-second products. The current transformer couples the entire transient current, including the DC component, to the secondary detection resistor for measurement, but it also requires the core to be properly reset every time the current pulse passes zero. Especially in average current mode control, current transformer detection is more applicable because the pulse current detected in average current mode control returns to zero in each switching cycle. [page]

In most control circuit topologies, the duty cycle is close to 100% when the current passes through zero, so the magnetic reset time when the current passes through zero accounts for only a small proportion of the switching cycle. Current transformer detection has a wide bandwidth while maintaining a good waveform. The current transformer also provides electrical isolation, and the detection current is small and the loss is small. The detection resistor can be a slightly larger value, such as a resistor of 10 to 20 ohms. In order to reset the magnetic core in a very short time, it is often necessary to add a large reverse bias to the current transformer, so when designing the current transformer circuit, a high-voltage diode should be used to couple between the secondary side of the current transformer and the detection resistor.

The solution to this problem is to use two current transformers to measure the switch current and the diode current respectively. As shown in Figure 4, the actual inductor current is the synthesis of these two currents, so that each current transformer has enough time to reset. However, it should be noted that the turns ratio of the two current transformers should be the same to keep the current on the detection resistor RS symmetrical.

3 Methods to prevent current detection circuit saturation

The power factor correction circuit generally uses a boost circuit and dual transformer detection, but when the line current passes through zero, the current transformer is also particularly prone to saturation. Because the duty cycle at this time is about 100%, it is easy to cause the magnetic core to not have enough time to reset. For this reason, some measures can be taken in the external circuit to prevent the current transformer from saturating. For example, the current amplifier output clamp is used to limit its output voltage, and further limit the duty cycle to less than 100%. The circuit is shown in Figure 5.

Current transformer detection is most suitable for symmetrical circuits, such as push-pull circuits and full-bridge circuits. For single-ended circuits, especially boost circuits, there will be some problems that we must pay attention to. Therefore, the current transformer cannot be used to directly measure the input current of the boost circuit, because the inductor current cannot return to zero and the DC value is "lost"; and the current transformer cannot be saturated due to magnetic reset, thus losing the overcurrent protection function, and the output generates overvoltage, etc. The same problem exists in the step-down circuit, and the current transformer cannot be used to directly measure the output current.

If the core of the current transformer cannot be reset, it will lead to core saturation. Current transformer saturation is a very serious problem. First, the current value cannot be measured correctly, so effective current control cannot be performed; second, the current error amplifier always "thinks" that the current value is less than the set value, which will cause the current error amplifier to over-compensate and cause current waveform distortion. For the boost circuit, the inductor current is the input current. Then, in the continuous current working mode, whether charging or discharging, the inductor current is always greater than zero, that is, a charging and discharging waveform is superimposed on the DC value.

Another factor to consider for the core reset of the current detection circuit is the leakage inductance and distributed capacitance of the secondary coil. In order to reduce losses, a current transformer with a larger turn ratio is generally selected, but the larger the turn ratio, the larger the leakage inductance and distributed capacitance of the secondary coil. And when the core is reset, the secondary inductance and distributed capacitance resonate. If the distributed capacitance is large, the resonant frequency is low and the period is long. Then, when the duty cycle is large and the core reset time is short, the secondary coil does not have enough time to release energy to reset the core. Therefore, try not to choose a current transformer with a large turn ratio.

If better characteristics are required or operation is required over a wide range, the circuit of Figure 6 can be used. This circuit will reversely adjust the clamping voltage according to the line voltage. The process of setting the clamping voltage is very simple. At the beginning of the startup, the current amplifier is clamped at a relatively low value (about 4V), and the system starts to work, but the zero-crossing error is large; once the system is working normally, the clamping voltage will increase, the current transformer is close to saturation, and the clamping voltage will rise to 6.5V at most (low voltage and large load) and the THD of the current will be within the acceptable range (<10%) to limit the maximum duty cycle. The set clamping voltage cannot be too low, otherwise the current zero-crossing distortion will be large.

Each current pulse resets the core to overcome the core saturation method. In addition to improving the external circuit, the current detection circuit can also be improved. Generally, the current detection circuit is self-reset, that is, the energy stored in the core and the open circuit impedance of the current transformer are used to generate enough volt-second product in a short time to reset. However, when the duty cycle is greater than 50%, especially close to 100%, there may not be enough time to reset the core. At this time, in addition to the current amplifier output clamp, a forced reset circuit can also be used. The leakage inductance affects the time of current rise and fall, and the distributed capacitance affects the bandwidth of the current transformer.

There are many circuits for forcing the core to reset, such as using additional coils or center-tapped coils, but the simplest method is to use the circuits shown in Figures 7 and 8 to force the core to reset. When a pulse current comes, there is no difference between the operation of the forced reset circuit and the self-reset circuit. When resetting, the current from VCC through Rr is added to the core reset current, the parasitic capacitor is quickly charged, the secondary voltage is reversed, the volt-second product increases, and the core reset speed is accelerated. If you need to get a negative detection voltage but don't want to use a negative voltage to force reset, use the circuit shown in Figure 8.

4. Droop effect of current transformer

The pulse current on the secondary side of the current transformer must be equal to the current on the detection resistor by subtracting a magnetizing current generated by the pulse voltage on the secondary side of the current transformer winding, which increases linearly with time from zero. The magnitude of the magnetizing current is:

(1) Where: US - secondary voltage; LS - secondary inductance; n - Ns/Np; Δt - current pulse width

As shown in formula (1), the larger the secondary inductance value, the smaller the droop effect; the smaller the turns ratio, the smaller the droop effect. However, it is best not to reduce the turns ratio by reducing the number of turns on the secondary side, because this will reduce the inductance of the secondary side. The number of turns on the primary side should be increased to reduce the turns ratio when space permits. Therefore, in order to obtain a larger secondary detection voltage, it should not be achieved entirely by increasing the value of the detection resistor Rs, but also by reducing the secondary droop effect to increase the secondary pulse current. At the same time, a large value of Rs will also make it difficult to reset the magnetic core. At the beginning, the secondary current is n times the primary current, but as time goes by, the magnetizing current increases and the secondary current drops sharply. This is the droop effect of the current transformer.

5 Experimental results

In the power factor correction circuit, the detection circuit shown in FIG4 is used, and the measures for preventing core saturation and reducing droop effect as described above are adopted. When the transformation ratio of the current transformer is 1:50, the secondary inductance is 30mH, the secondary voltage is 2V, and the current wave pulse width is 5μs, the following is obtained:

Compared with the detection current of more than ten amperes, the current reduction effect is not obvious.

6 Conclusion

In order to reduce losses, current transformer detection is often used. Current detection plays an important role in current control. In the design of the current transformer detection circuit, the saturation problem of the current transformer and the droop effect of the secondary current are comprehensively considered to select the appropriate core reset circuit, turns ratio and detection resistor.