Convert a constant power source into a constant current source

Fault protection is an important function that all power supply controllers have. Almost all applications require overload protection. For peak current mode controllers, this function can be easily achieved by limiting the maximum peak current. In a discontinuous flyback structure, setting a limit on the peak current ultimately limits the power that the power supply can obtain from the input source. However, limiting the input power does not limit the output current of the power supply. If the input power remains unchanged during an overload fault, the output current increases (P=V*I) as the output voltage drops. In the event of a short circuit fault, this will cause unacceptably high losses in the output rectifier or system power distribution. With some minor innovations and a few additional components, this article shows you how to improve a simple peak current limit to turn the power supply into a constant current source instead of a constant power source.

Figure 1 compares the ideal output voltage and current for constant power and constant current limiting. In both cases, overload fault protection operates at 120% of the maximum rated load. In a system using power limiting, the output current increases as the load increases and the voltage reverses. In a real system, a flyback controller with power limiting will shut down at some point due to bias losses in the controller. In contrast, a system with current limiting shuts down immediately once the overload threshold is exceeded. Current limiting can be achieved by directly sensing the load current on the secondary side of the isolation boundary. However, this requires more circuitry, is less efficient, and is generally prohibitively expensive.

Figure 1 Ideal power limiting generates high current, triggering fault protection

Figure 2 shows the schematic of a 5V/5W discontinuous flyback power supply used in a mobile device charger. In this example, we use the UCC28C44 controller, which is representative of the most economical peak current mode controllers with power limiting. In the discontinuous flyback structure, if the efficiency effect is ignored, the load power (P) can be calculated using Equation 1.

Since the transformer inductance (L) and switching frequency (f) are fixed, the output voltage (VOUT) can be regulated 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.

Figure 2 This 5V/5W reverse circuit achieves power limitation by limiting the peak transformer current.

Inside the flyback converter, the feedback voltage at pin 1 (COMP) is compared to the peak current. This peak current is sensed by R15 and filtered by R13 and C12. If the current sense voltage reaches over 1V, a separate 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. If the power is held constant, Equation 1 can be rewritten as Equation 2. In this equation, we can clearly see that the output current is inversely proportional to the output voltage when power is limited.

Some controllers also include a second comparator. When the peak current exceeds the first comparator, the second comparator trips open. This second comparator triggers the controller to shut down completely and initiate a restart cycle. This extra level of protection is designed to protect the power supply itself from catastrophic failures, such as shorted transformer windings or shorted output diodes. However, most situations involving shorted loads will not exceed this threshold.

Figure 3 shows the output and bias voltage versus load current for the circuit shown in Figure 2. The output VI characteristics are very close to the ideal case shown in Figure 1. Power limiting begins at about 1.3A of load current. 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 shutdown level, the PWM controller shuts down.

Figure 3: After the bias voltage drops below the controller shutdown threshold, the converter no longer provides power-limited current.

[page]In this example, although the peak current limit activates when the load exceeds 1.3A, the load current can reach more than twice the rated load before the converter shuts down. In some applications, this is unacceptable. Instead, a more square VI curve is more desirable. This VI curve can be easily achieved by taking advantage of the property that the bias voltage decreases as the load increases beyond the power limit point. With just a few additional components, the switching frequency can be folded during power limiting using the decreasing bias voltage. By doing this, the switching frequency is forced to be proportional to the output voltage, as shown in Equation 3. Substituting Equation 3 into Equation 2, we see that, in theory, the output current during power limiting is no longer dependent on 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. The internal oscillator is programmed to set the switching frequency of the flyback converter via R10, R8, and C11. An internal 5V source charges C11 via R10 and R8. As the bias voltage drops, the resistor divider of R7 and R11 turns Q1 on and takes precedence over the internal 5V source, reducing the switching frequency. The bias diode (D4) must now be a dual series diode so that R7 and R11 do not redirect the controller's current during startup. The values ​​of R7 and R11 are chosen so that Q1 is off during normal operation and only turns on when the bias voltage drops below approximately 12V.

Figure 4 Adding five discrete components can enhance the power limiting function and reduce the maximum fault current.

The result of adding these components is shown in Figure 5. As before, both the output voltage and the bias voltage begin to drop as the power supply enters power limit. Once the bias voltage drops enough to turn on Q1, any further increase in load current causes the switching frequency to decrease, which in turn reduces the effective power delivered to the load. This speeds up the overcurrent shutdown process. Note that there is still some degree of correlation between the output current and the output voltage due to the bias winding coupling inside the transformer and the finite gain of Q1. Despite these drawbacks, the added circuitry greatly improves the VI characteristic. In fact, the power supply will now not deliver more than 1.5A to the faulty load.

Figure 5 The VI curve of the power supply using the enhanced power limiting circuit shows that the output voltage shows a significant drop in the overload condition

In summary, a power supply with power limit protection can still provide plenty of current to an overloaded output. As shown in this article, accurate current limit functionality can be easily and inexpensively implemented by adding only a few components around the primary-side controller. Although it is targeted at flyback converters, this approach can also reduce excess current in buck converters.