Abstract: This paper introduces the implementation method of a current detection circuit and discusses the common problems encountered in current detection, such as current transformer saturation and secondary current droop. Finally, experimental results are used to analyze the current detection method in a boost circuit. Keywords: Current detection, current transformer, magnetic core reset, power switch circuit. The circuit topology of the power switch circuit is divided into current-mode control and voltage-mode control. Current-mode control has advantages such as fast dynamic response, simplified compensation circuit, large gain bandwidth, small output inductance, and easy current sharing, and therefore has been increasingly widely used. In the current-mode control circuit, accurate and efficient measurement of the current value is required, so the implementation of the current detection circuit becomes an important issue. This paper introduces the implementation method of the current detection circuit and discusses the common problems encountered in current detection, such as current transformer saturation and secondary current droop. Finally, experimental results are used to analyze the current detection method in a boost circuit. 2 Implementation of the Current Detection Circuit In the control circuit of the current loop, the current amplifier usually chooses a large gain. The advantage of this is that a smaller resistor can be selected to obtain a sufficient detection voltage, and the loss is also small when the detection resistor is small. There are two main methods for implementing current sensing circuits: resistive sensing and current sense transformer (FST) sensing. Resistive sensing has two types, as shown in Figures 1 and 2. When directly sensing the current of the switching transistor using Figure 1, a small RC filter circuit must be connected in parallel with the sensing resistor RS, as shown in Figure 3. This is because when the switching transistor is turned off, the collector capacitor discharges, generating a transient current spike on the current sensing resistor. The pulse width and amplitude of this spike are often sufficient to lock the current amplifier, causing the PWM circuit to malfunction. However, in practical circuit design, especially in high-power, high-current circuits, resistive sensing is not ideal because the sensing resistor has high losses, reaching several watts or even tens of watts; and it is difficult to find resistors as small as a few hundred milliohms or tens of milliohms. In practice, current sense transformer (FST) sensing is more practical in high-power circuits, as shown in Figure 4. FST sensing maintains a good waveform while having a wide bandwidth. The current transformer also provides electrical isolation, and the loss is low for small sensing currents. A slightly larger value can be used for the sensing resistor, such as a resistor of ten or twenty ohms. A current transformer measures the entire transient current, including the DC component, coupled to the sensing resistor on the secondary side. However, it also requires the core to reset correctly each time the current pulse crosses zero, especially in average current mode control, where current transformer sensing is more suitable because the detected pulse current returns to zero in each switching cycle. To ensure complete magnetic reset of the current transformer, the core needs an equal and opposite volt-second product. In most control circuit topologies, the duty cycle is close to 100% when the current crosses zero, so the magnetic reset time at zero current crossover accounts for only a small proportion of the switching cycle. To reset the core in such a short time, a large reverse bias voltage is often required across the current transformer. Therefore, a high-voltage diode should be used to couple between the secondary side of the current transformer and the sensing resistor when designing the current transformer circuit. 3. Methods to Prevent Saturation of the Current Sensing Circuit If the core of the current transformer cannot reset, it will lead to core saturation. Current transformer saturation is a serious problem. Firstly, it prevents accurate current measurement, hindering effective current control. Secondly, it causes the current error amplifier to consistently "interpret" the current as less than the set value, leading to overcompensation and waveform distortion. Current transformers are best suited for symmetrical circuits, such as push-pull and full-bridge circuits. However, in single-ended circuits, especially boost circuits, several issues arise. In boost circuits, the inductor current is the input current. Therefore, in continuous current operation, regardless of charging or discharging, the inductor current is always greater than zero, resulting in a charging/discharging waveform superimposed on the DC value. Consequently, current transformers cannot be used to directly measure the input current of boost circuits because the inductor current cannot return to zero, causing the DC value to be "lost." Furthermore, the inductor saturates due to its inability to magnetically reset, losing overcurrent protection and causing overvoltage at the output. The same problem exists in buck circuits; current transformers cannot be used to directly measure the output current. One solution to this problem is to use two current transformers to measure the switching current and diode current respectively, as shown in Figure 4. The actual inductor current is the sum of these two currents, giving each current transformer sufficient time to reset. However, it's crucial that the turns ratio of both current transformers be identical to maintain symmetrical current across the sensing resistor RS. Power factor correction circuits typically employ boost circuits with dual current transformers for detection, but current transformers are particularly prone to saturation when the line current crosses zero. This is because the duty cycle is approximately 100% at this point, easily causing insufficient time for the magnetic core to reset. Therefore, measures can be taken in the external circuit to prevent current transformer saturation. For example, a current amplifier output clamp can be used to limit its output voltage and further limit the duty cycle to less than 100%, as shown in Figure 5. Setting the clamping voltage is straightforward. At startup, the current amplifier clamps at a relatively low value (approximately 4V) as the system begins operation, but the zero-crossing error is significant. Once the system is operating normally, the clamping voltage will increase, and the current transformer will approach saturation. The clamping voltage can rise to a maximum of 6.5V (under low voltage and high load conditions), and the current THD will be within an acceptable range (<10%) to limit the maximum duty cycle. The clamping voltage should not be set too low, otherwise, it will cause large current zero-crossing distortion. For better characteristics or operation over a wider range, the circuit shown in Figure 6 can be used, which adjusts the clamping voltage in reverse according to the line voltage. Each current pulse resets the core to overcome core saturation. Besides improving the external circuit, the current sensing circuit can also be improved. Generally, a self-resetting current sensing circuit is used, utilizing the energy stored in the core and the open-circuit impedance of the current transformer to generate a sufficient volt-second product in a short time for reset. However, when the duty cycle is greater than 50%, especially close to 100%, there may not be enough time for the core to reset. In this case, in addition to clamping the current amplifier output, a forced reset circuit can be used. There are many circuits for forced core reset, such as those using an additional coil or a center-tapped coil, but the simplest method is to use the circuits shown in Figures 7 and 8. When a pulse current arrives, the forced reset circuit and the self-reset circuit work indistinguishably. When resetting, the current from VCC through Rr is added to the core reset current, the parasitic capacitance charges rapidly, the secondary voltage reverses, the volt-second product increases, and the core reset speed increases. If a negative detection voltage is required but a negative voltage is not desired for forced reset, the circuit shown in Figure 8 can be used. Another factor to consider for core reset in a current detection circuit is the leakage inductance and distributed capacitance of the secondary coil. To reduce losses, a current transformer with a large turns ratio is generally chosen, but a large turns ratio results in a large leakage inductance and distributed capacitance of the secondary coil. The leakage inductance affects the rise and fall time of the current, while the distributed capacitance affects the bandwidth of the current transformer. Furthermore, during core reset, the secondary inductance and distributed capacitance resonate. If the distributed capacitance is large, the resonant frequency is low and the period is long. Therefore, 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. So, current transformers with too large a turns ratio should be avoided as much as possible. 4. Droop effect of current transformers The pulse current on the secondary side of the current transformer must be reduced by the magnetizing current generated by the pulse voltage on the current transformer winding, which starts from zero and increases linearly with time, to equal the current on the sensing resistor. The magnitude of the magnetizing current is: Idroop=nUs / Ls·△t (1) Where: US——secondary voltage LS——secondary inductance n——Ns/Np Δt——current pulse width At the beginning, the secondary current is n times the primary current, but as time increases, the magnetizing current increases, and the secondary current drops sharply. This is the droop effect of the current transformer. Therefore, to obtain a larger secondary detection voltage, it is not possible to rely solely on increasing the value of the detection resistor Rs. It is also necessary to reduce the secondary droop effect to increase the pulse current of the secondary side. At the same time, a large value of Rs will also make it difficult for the magnetic core to reset. As shown in equation (1), the larger the secondary inductance, 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 turns ratio should be reduced by increasing the number of turns on the primary side if space permits. 5 Experimental Results In the power factor correction circuit, using the detection circuit shown in Figure 4, and adopting the measures to prevent magnetic core saturation and reduce droop effect as described above, when the current transformer ratio is 1:50, the secondary inductance is 30mH, the secondary voltage is 2V, and the current pulse width is 5μs, we get: Compared with the detection current of more than ten amperes, the current drop effect is not obvious. 6. Conclusion Current sensing plays a crucial role in current control. Current sensing can be categorized into resistance sensing and current transformer sensing. To reduce losses, current transformer sensing is commonly used. In designing a current transformer sensing circuit, the impact of circuit topology on the sensing effect must be fully considered, along with the saturation problem of the current transformer and the droop effect of the secondary current, to select an appropriate core reset circuit, turns ratio, and sensing resistor.