User`s guide

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Software Design

3.5.2 Soft-Start Routine

Each individual stage of the SMPS AC/DC Reference Design employs a controlled

soft-start routine. At power-up, all reference set points are configured to produce 0V

output. Once the power-on delay has lapsed, the outputs begin their soft-start where

the reference set point is incremented until the desired output voltage is reached.

3.5.3 Overtemperature Protection

Temperature sensors are provided on the SMPS AC/DC Reference Design in two

positions. Overtemperature protection must be enabled to prevent damage to the

system in the event of:

• Insufficient airflow in the system caused by a failure of the cooling fan

• Operation of the system at a high ambient temperature

The implementation detail for each sensor is described in the following sections.

3.5.3.1 PCB OVERTEMPERATURE PROTECTION

One of the PCB temperature sensors is located on the secondary side in the middle of

the four Buck phases. The other temperature sensor is located on the primary side just

below the Boost inductor (L4). An analog temperature sensor is chosen that outputs an

analog voltage proportional to the measured temperature.

The output of the temperature sensor is connected to analog input AN8 on the

secondary side dsPIC DSC, and AN10 on the primary side dsPIC DSC. The PCB

temperature is measured in the ADC ISR and checked for overtemperature protection

in the fault loop. The maximum temperature set point is configured to approximately

90°C. If the measured temperature exceeds the maximum set point, a fault is

generated and the PWM outputs are turned OFF.

3.5.4 Input AC Undervoltage/Overvoltage Protection

The PFC Boost Converter is designed to operate normally for input voltages in the

range 85V–265V. In the event of an undervoltage condition, the circuit components will

undergo excessive stress due to the additional current drawn by the system to deliver

maximum output power. Therefore, undervoltage and overvoltage faults must be

implemented to prevent damage to the system and load. In the event of an overvoltage

condition, the input voltage may exceed the device ratings.

There are instances when the power grid may exhibit momentary voltage fluctuations,

which must be ignored by the system.

The input AC voltage protection is implemented in software by calculating the average

input voltage in the ADC ISR. The average input voltage is calculated as a part of the

PFC control scheme, and is checked in the fault loop to detect a sustained

undervoltage/overvoltage condition.

The first time an undervoltage/overvoltage condition is detected, a counter is

incremented. If the undervoltage/overvoltage condition remains for an extended period

of time, a fault is generated and the outputs are turned OFF.

3.5.5 Fault Source Identification

If a Fault condition is detected, it is often required that the system be turned OFF to

prevent damage. The PWM module on the dsPIC33FJ16GS504 has a built-in latched

fault mode. Using the latched fault mode, certain faults will immediately disable the

PWM outputs with no software overhead.

Each fault is assigned a fault ID to indicate the source of the last fault that occurred. A

visual indication is also provided using LEDs on both the primary and secondary side

of the SMPS AC/DC Reference Design.