Extremely compact and energy-saving instantaneous high power output solution

Motor loads such as hand tools, inkjet printers and hard disk drives require a higher starting current at the moment of startup. Therefore, the power supply of such products must be able to provide a higher peak power output at the moment of startup, but the power required during normal working conditions is much lower than the power required at the moment of startup. Faced with such a wide range of power requirements, if the transformer is designed based on the conditions of the moment of startup, it will cause overdesign in normal working conditions and increase design costs. Therefore, the trade-off between application requirements and design costs often causes design difficulties. In order to optimize the design of these requirements, we will analyze the characteristics and principles of the requirements below.

According to Faraday's law, the energy conversion formula of the flyback converter is shown in formula (1). From the following formula, we can see that the peak current change on the primary side is affected by parameters such as input voltage (VI), inductance (L), on-time (DON) and its operating frequency (FSW). In the above example, a larger starting current (IP) is required at the moment of startup, so the switch on-time will increase with the load, causing the peak current change (DIP) to increase accordingly. At this time, the magnetic flux density may exceed its upper limit (BMAX) due to the excessive peak current change, causing the transformer to saturate and produce a short circuit phenomenon, resulting in a rapid increase in the peak current on the primary side and damage to the surrounding components, as shown in Figure 1.

Primary side peak current change

VI Input Voltage

DON switch on time

L Primary side inductance

FSW Off-Off switching frequency

Figure 1. Transformer saturation phenomenon

From formula (1), we can see that the change in peak current is inversely proportional to the switching frequency, as shown in formula (2). If the switching frequency is increased as the load increases (Figure 2), the change in peak current can be effectively suppressed, which can not only provide instantaneous high wattage output and improve instantaneous starting capability, but also reduce the magnetic flux of the transformer at the moment of starting, reduce component size and reduce cost.

Figure 2. Adjusting the switching frequency according to the load.

Currently, there is no PWM IC solution specifically for this application on the market. If you want to meet the above requirements, you need to add peripheral application circuits. Therefore, the following uses a product that meets such requirements - LD7533 for illustration.

LD7533 is a current mode control IC that integrates protection functions such as overvoltage protection (OVP), overtemperature protection (OTP) and overload protection (OLP). It also has adjustable soft start (Soft STart), undervoltage lockout (UVLO), built-in output PWM frequency jump and leading edge surge shielding (LEB) functions. In addition, LD7533 has low startup current (<18μA) and extremely low operating current (<1mA) when operating under light load, which can achieve more streamlined and energy-saving requirements than traditional current mode control design.

Taking the 18W (12V/1.5A) application as an example, when the Peak power requirement reaches 36W (12V/3A), the LD7533 increases the original PWM frequency from 62KHz to 180KHz to avoid transformer saturation. This not only meets the instantaneous higher wattage requirement, but also effectively reduces the maximum magnetic flux density (BMAX) by 21%, as shown in Figure 3 below.

Figure 3. Comparison between LD7533 and general PWM IC

This LD7533 also provides the function of adjusting the soft start time and overload protection delay time to meet the application requirements of various products.

As shown in Figure 4, by adjusting the capacitance on the CT pin, the following parameters can be adjusted:

Soft-Start time: effectively reduces the spike voltage on the MOSFET.

Overload protection delay time (OLP delay time): to meet various Peak power time requirements while taking into account the safety of system applications.

Figure 4. Soft start time and overload protection delay time adjustment

Application Examples

If traditional PWM IC is used to realize the above functions, many peripheral circuits are often needed, which not only increases the design time and material cost, but also the circuit reliability is a big problem. As shown in Figure 5, LD7533 successfully integrates the circuits required for the application and provides a very simplified design solution, allowing engineers to complete product design more quickly.

Figure 5: Comparison between LD7533 high power output solution and traditional applications

in conclusion

Using the LD7533 instantaneous high power output solution, the experimental results show that instantaneous high power output can be achieved and the transformer design cost can be reduced, achieving the best balance between application requirements and design costs. In addition, if its soft start time and overload protection delay time are adjusted, the application requirements of various products can be met.

With low startup current (<18μA) and very low operating current (<1mA) during light load operation, this product is more compact and energy-efficient compared to traditional current mode control, and complies with relevant energy-saving regulations of various countries such as Energy Star.

From the perspective of the high-reliability peripheral application circuit integration solution provided by LD7533, the simplified circuit can effectively reduce the design time of engineers and reduce design costs such as materials, thus significantly shortening the time required for project development.