Improved method for a simple peak current limit
Fault protection is an important feature of all power controllers. Almost all applications require overload protection. For peak current mode controllers, this can be easily accomplished by limiting the maximum peak current. In a discontinuous reverse configuration, setting a limit for the peak current can ultimately limit the power that the power supply draws from the input source. However, limiting the input power does not limit the output current of the power supply. If the input power remains the same in the event of an overload fault, the output current increases as the output voltage drops (P = V * I). In the event of a short circuit fault, this can cause unacceptably high losses in the output rectifier or system power distribution. This article uses some small innovations and several additional components to show you how to improve a simple peak current limit, turning the power supply into a constant current source instead of a constant power source.
Figure 1 compares the ideal output voltage with a constant power and constant current limit current. In both cases, the overload fault protection works at 120% of the maximum rated load. In a system that uses power limiting, the output current increases as the load increases in reverse voltage. In real-world systems, a power-limited reverse controller will turn off at some point due to the controller's bias loss. In contrast, once the overload threshold is exceeded, the system with current limiting will shut down immediately. Current limiting can be achieved by directly detecting the load current on the secondary side of the isolation boundary. However, doing so requires more circuitry, less efficiency, and the cost is generally prohibitively high.
Figure 2 shows a schematic of a 5V/5W non-continuous reverse power supply used by a mobile device charger. In the example, we used the UCC28C44 controller, which is representative of most economical peak current mode controllers with power limiting. In a discontinuous reverse structure, if the efficiency effect is ignored, Equation 1 can be used to calculate the magnitude of the load power (P).
Since the transformer inductance (L) and the switching frequency (f) are fixed, the output voltage (VOUT) can be adjusted by controlling the peak primary current (IPK). As the output current (IOUT) increases, the voltage begins to drop, but the feedback loop requires a higher peak current to maintain voltage regulation.
Inside the inverter, the feedback voltage at pin 1 (COMP) is compared to the peak current. This peak current is detected by R15 and filtered using R13 and C12. If the current sense voltage reaches 1V, the individual overcurrent comparator terminates the pulse. This peak current limiting method is the same as the power limiting process in most pulse width modulation (PWM) controllers. Equation 1 can be rewritten as Equation 2 if the power remains constant. In this equation, we can clearly see that the output current is inversely proportional to the output voltage when the power is limited.
Some controllers also include a second stage comparator. When the peak current is higher than the first stage comparator, the second stage comparator trips open. This second stage comparator triggers the controller to shut down completely and initiates a restart cycle. The purpose of this additional protection stage is to prevent catastrophic failure of the power supply itself, such as shorting the transformer windings or shorting the output diodes. However, most situations involving short-circuit loads generally do not exceed this threshold.
Figure 3 shows the comparison of the output and bias voltages with the circuit load current shown in Figure 2. The output VI characteristics are very close to the ideal case shown in Figure 1. The power limit is started when the load current reaches approximately 1.3A. As the load increases, the output voltage begins to drop. Since the bias voltage is a reflection of the output voltage, it also begins to drop. When the bias voltage drops below the 9V turn-off level, the PWM controller turns off.
In this example, although the peak current limit is activated when the load exceeds 1.3A, the load current will be more than twice the rated load before the converter is turned off. In some applications, this is unacceptable. Conversely, a more square VI curve is ideal. With the increase in load beyond the power limit point, the bias voltage drops, and with this feature, we can easily obtain this VI curve. With just a few more components, you can use the decreasing bias voltage to fold the switching frequency during power limiting. After doing so, the switching frequency is forced proportional to the output voltage, as shown in Equation 3. Substituting Equation 3 into Equation 2, we find that theoretically the output current during power limitation is no longer dependent on the magnitude of the output voltage, see Equation 4.
Some of the components added to create this improved current limit are highlighted in the schematic shown in Figure 4. Program the internal oscillator and set the switching frequency of the inverter with R10, R8 and C11. An internal 5V source charges C11 through R10 and R8. As the bias voltage drops, the resistor dividers of R7 and R11 turn on Q1 and control over the internal 5V source to reduce the switching frequency. The bias diode (D4) must now be a double series diode so that R7 and R11 do not divert the controller current during startup. The values ​​of R7 and R11 need to be properly selected so that Q1 is turned off during normal operation and only turned on when the bias voltage drops below approximately 12V.
The result of adding these components is shown in Figure 5. As before, both the output voltage and the bias voltage begin to drop when the power supply enters the power limit. Once the bias voltage drops to a level sufficient to turn on Q1, any continued increase in load current will cause the switching frequency to decrease, which in turn will reduce the effective power supplied to the load. This will speed up the overcurrent shutdown process. Note that there is still a degree of correlation between the output current and the output voltage due to the bias winding coupling inside the transformer and the limited Q1 gain. Despite these shortcomings, the added circuitry has greatly improved the VI characteristics. In fact, the power supply now does not supply more than 1.5A to the faulty load.
In summary, a power supply with power-limiting protection can still provide a large amount of current for the overload output. As described in this article, precise current limiting is achieved easily and at low cost by simply adding a few components around the primary side controller. Although it is aimed at a reverse converter, this scheme can also reduce the excess current of the buck converter.
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