Purpose of external I limit pin
What exactly is the purpose of the external current limit pin of topswitch GX devices? I may have been misinterpreting it as a means of controlling the output current which is doubtful because a lot of design using this topswitch family still uses separate current limit circuit (that uses sense resistor and bjt).
Regards,
Emman
Comments
The "X" pin on TOPSwitch GX limits the maximum drain current for the device. If the X pin is connected to the Source pin, the maximum drain current is allowed. If a resistor is placed between the X pin and Source pin, the current limit is reduced (see figure 54b in the datasheet for current limit vs. resistance). The primary reason of this function is to be able to use a larger device but maintain the same current limit to improve efficiency and lower the device temperature. Since this device regulates using PWM, the output current limit is not accurately controlled by the primary drain current. Thus a secondary current sense circuit is required to accurately limit the output current.
Z. Cochrane
The "X" pin on TOPSwitch GX limits the maximum drain current for the device. If the X pin is connected to the Source pin, the maximum drain current is allowed. If a resistor is placed between the X pin and Source pin, the current limit is reduced (see figure 54b in the datasheet for current limit vs. resistance). The primary reason of this function is to be able to use a larger device but maintain the same current limit to improve efficiency and lower the device temperature. Since this device regulates using PWM, the output current limit is not accurately controlled by the primary drain current. Thus a secondary current sense circuit is required to accurately limit the output current.
Z. Cochrane
Oh I see, thank you sir :). I was able to design a flyback charger before rated at 28V 2A and uses TOP227 TopSwitch. It also has a current limit that uses BJT and current sense resistor. It's short circuit current is fairly at 2A also. I designed a new charger that uses TOP249. This time, it is rated at 28V 4.5A. The voltage starts to drop at >4.5A but when you short circuit the output, the current drawn is 12A-14A (i think it hiccups). What may be the reason for not being able to maintain short circuit current at 4.5A? Does changing the TopSw have an effect? the old and new circuit is almost the same except for the topsw and transformer. Thank you very much.
Computer power supplies are rated based on their maximum output power. Typical power ranges are from 300 W to 500 W lower than 300 W for Small aldo shoes form factor systems and are intended for ordinary home computers, the use of which is limited to Internet-surfing and burning and playing DVDs. Power supplies used by gamers and enthusiasts mostly range from 450 W to 1400 W. Typical gaming PCs feature power supplies in the range of 500-800 W jordan shoes, with higher-end PCs demanding 800-1400 W supplies. The highest-end units are up to 2 kW strong and are intended mainly for servers and, to a lesser degree, extreme performance computers with multiple processors, several hard disks and multiple graphics cards. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer shoes. A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. Therefore it is possible to overload a PSU on one rail without having to use the maximum rated power.
The "hiccups" is what we call auto-restart. This happens when the feedback current is below the minimum threshold (around 2 mA). This feature keeps the power supply from delivering full power when the feedback has an open fault.
The reason that the power supply provides such high current when the output is shorted is because the transformer secondary voltage is near zero volts. During the switching time of auto-restart, the primary current limit is what limits the output power. The TOP249 current limit is 5.4 Amps. The primary current will be slightly higher due to the high di/dt (during a short) and the internal delay of the current limit shutdown circuit. This peak drain current will be multiplied by the transformer's turns ratio to deliver current to the output. This is why you see such a high peak current in the shorted output. However the average output current is much less due to the low duty cycle of the auto-restart function.
If the peak output current is a problem and since you mentioned that the output voltage starts to drop with Iout>4.5A. If there is enough margin you can use a resistor on the X pin to reduce the maximum output power (figures 54A and 54B in the datasheet will help you select the proper value). This will reduce the peak output current during a short. If you are unable to deliver the desired output power while keeping the peak short circuit current low enough, then you will need to add a secondary current limit circuit.
Z. Cochrane
The "X" pin on TOPSwitch GX limits the maximum drain current for the device. If the X pin is connected to the Source pin, the maximum drain current is allowed. If a resistor is placed between the X pin and Source pin, the current limit is reduced (see figure 54b in the datasheet for current limit vs. resistance). The primary reason of this function is to be able to use a larger device but maintain the same current limit to improve efficiency and lower the device temperature. Since this device regulates using PWM, the output current limit is not accurately controlled by the primary drain current. Thus a secondary current sense circuit is required to accurately limit the output current.
Z. Cochrane