The ICE3BRxx65J PWM controller is the latest in the series of CoolSET-F3 (F3R). Particularly suitable for low power applications from 9 to 30W such as DVD player/recorders, set-top boxes, adapters, auxiliary supplies, etc., this tiny, versatile device has an active burst mode that enables it to achieve the best-in class low standby power performance. Its frequency jittering feature effectively reduces EMI noise. Its propagation delay compensation provides an internal precise peak current limitation for a wide input range. It also provides all the necessary protection against open loop, over-load, Vcc over-voltage, over-temperature, etc. To enhance over-load protection, flexible blanking time features provide short periods of maximum output power.
With all these features in a small DIP-8 package, the ICE3BRxx65J is a complete solution for low power applications. A full-featured power supply control is done by just adding a few capacitors and resistors (Figure 1).
Key functions
Active burst mode
To obtain very low standby power, the ICE3BRxx65J needs to minimize at both conduction and switching losses at light load. Its BiCMOS design has a start-up cell that can effectively reduce power consumption. However, major power loss comes from switching. The active burst mode is employed to reduce switching frequency by skipping switching cycles. The burst mode scheme is very robust and enables the system to achieve an extremely low standby power. The measured standby power at no load is 25mW (Figure 2).
During light load operation, the feedback voltage drops with the load. When the VFB voltage falls below 1.35V for 20ms, the ICE3BRxx65J enters the active burst mode, upon which, the feedback control level is shifted to operate between 3.0 and 3.5V. The system will stop switching when the feedback voltage hits 3.0V, when the output voltage is still within regulation. When the feedback voltage increases and hits 3.5V, the system starts switching, which means the output voltage is dropped to the lower limit of regulation. During the burst on period, the current sense limit is reduced to 1/3 of the normal control, which can effectively reduce the audible noise. When the output loading increases to nominal load, the feedback voltage will rise with the output voltage drops. When it hits 4.0V, the system is released from active burst mode and recovers to normal operation (Figure 3). Since the ICE3BRxx65J is continuously monitoring the feedback voltage, the response on the load jump is fast and with minimum output drops.
Switching frequency modulation
The purpose of implementing the switching frequency modulation is to obtain good EMI performance. An EMI plot of 12W system employed with ICE3BR4765J is shown in Figure 4 (left). Compared to a non-frequency jittering version controller, ICE3B0565 in Figure 4 (right), it can show 5~10dB lower in average data measurement. The principle for the frequency jittering is to even out peak energy in particular frequencies. The selected frequency spread width is +/-4% of the switching frequency; 65kHz (± 2.6kHz) and the switching modulation period is 4ms (250Hz).
The frequency modulation, achieved by an internal saw tooth signal which being generated by a digital method, controls the jittering pattern of the switching clock such that the switching frequency reaches maximum at trough, and decreases linearly to the minimum at the crest of the saw tooth waveform (Figure 5).
Propagation delay compensation
Current mode PWM control employs peak current control. However, peak current control is inaccurate with the changes of input voltage. When the peak current is hit, there is a propagation delay time due to the internal logic circuit before the switching pulse stops. This propagation delay time is fixed in the IC controller but the peak current is varied with current rise time; ΔI/Δt, which can be varied by 3 times between low input line and high input line. The peak current control becomes inaccurate along a wide input voltage range. And thus the maximum power control is also inaccurate.
The ICE3BRxx65J, implemented with propagation delay compensation circuit, can effectively reduce the peak current inaccuracy. A demo board with ICE3BR4765J is measured, and the maximum peak output power is about 3% along a wide input voltage (Table 1).
The controlled peak current must have offset from the pre-set peak current limit. The degree of offset will depend on the rise time of the current; a faster current rate will have a larger offset, and a slower one will have less. A very important relationship reveals that the faster current rate has smaller duty ratio, and slower one has larger duty ratio. It is employed as the basis of the propagation delay compensation. The compensation scheme varies the peak current limit with a different duty ratio such that a faster current rate condition has a lower peak current limit, and a slower one has a higher current limit (Figure 6). With this compensation, the maximum power is almost independent of the input voltage.
Protection modes
Protection is one of the major factors to determine whether the system is safe and robust. The ICE3BRxx65J provides all the necessary protection including Vcc over-voltage, over-temperature, and over-load protection. Should any of such faults be found, the system will stop for a short time and then restart. If the fault persists, the system will stop again. When the fault is removed, the system resumes normal operation. Table 2 shows a list of protection and the failure conditions.
· Blanking time for over-load protection
Over-load protection is very important for a power supply as it would protect the power supply from over-heating after prolonged over-loads. However, if the timing to protect against over-loads is too fast, it will not be flexible enough for the application. The ICE3BRxx65J provides a blanking time scheme that enables protection after the desired blanking time has elapsed therefore offering sufficient protection while maintaining a certain degree of flexibility.
The blanking time scheme is divided into two modes; the basic mode and the extended mode. In basic mode, the blanking time is set at 20ms; i.e. system will go to protection after 20ms blanking time. In the extended mode, the blanking time can be increased by adding an external capacitor (CBK) at BA pin in addition to the basic mode; i.e. overall blanking time = basic + extended.
When there is an over-load fault, the feedback (FB) voltage will go up and hits 4.0V. The blanking time scheme is then activated. The scheme will first go to the basic mode; 20ms. If there is no CBK capacitor at BA pin, the voltage at BA pin will charge up immediately from 0.9 to 4.0V by an internal current source; 13uA. This will then trigger the auto-restart protection. If there is a CBK capacitor at BA pin, the protection can only be triggered when the BA pin hit 4.0V with the additional charging time from 0.9 to 4.0V by 13uA. This period of charging is called the extended blanking time.
Figure 7 shows the waveform for basic mode (left) and extended mode (right) blanking time for over-load protection.
· Auto-restart enable pin (BA pin)
Should the listed protections not be sufficient for an application, pulling down the BA pin voltage to less than 0.3V can activate a protection-enable pin.
A particular AC brownout feature can be achieved by making use of the BA pin. A hysteresis circuit in the dotted block of Figure 8 is activated if the falling voltage of the bulk capacitor (hits 100V) is sensed. Then the BA voltage will be pulled down and the system will stop switching immediately. When the bulk capacitor rises to 120V, the BA voltage is released and switching pulse returns to normal.
Click here for Illustrations:Figure 1Figure 2Figure 3Figure 4Figure 5Figure 6Figure 7Figure 8Table 1
