Capacitor charging power supplies are used for pulsed-power applications such as IPL or pulsed lasers for aesthetic medicine and for driving pulsed lasers intended for diverse medical or industrial applications, among many other application fields.
 

Key parameters for choosing a capacitor charging power supply
 

The most important parameters which will determine which capacitor charging power supply you will choose and eventually integrate into your product are:
 

• Electrical parameters

• Physical size

• Reliability

• Price
   

Among many, the main electrical parameters for choosing a capacitor charger are the following:
 

1. Output power: measured in Joules/Sec (or Watts). It determines how quickly the capacitor will be charged to the required operating voltage.
    

2. Repetition rate:  indicates how many times per minute or per second the capacitor can be charged. The higher the rate, the higher the average power the capacitor charging power supply delivers.
    

3. Output voltage rating: Indicates the voltage to which the load capacitor will be charged.
    

4. Input leakage current: a safety parameter indicating the leakage current to Earth. For instance, for most medical applications the leakage current should usually be 0.5mA or less, depending on the device class and required certification.
    

5. User interface, which usually includes as a minimum: Enable/Disable signal for activating the charging power supply, a "Vset" analog control signal for setting the capacitor voltage at the end of charge, an "End-of-Charge" signal, indicating the capacitor is fully charged, and an alarm signal, indicating a malfunction. 
    

Common capacitor charging power supplies technology
 

The technology used in the capacitor charging power supply has an impact on all the above parameters, especially on its physical size, its reliability and price.
 

In this paper we will discuss the standard, widely used constant current capacitor charging approach, and we will compare it to Advice Electronics Ltd.'s approach, which allows for smaller size, more reliable and cost-effective quasi-constant-power (QCP) capacitor charging power supplies.
 

Standard constant current capacitor charging approach
 

In the range of 1 or 2 KJoule/Sec. and above, most capacitor charging power supplies in the market are built using a constant current capacitor charging approach.
 

A bit of theory: the voltage (in Volts) across a capacitor is equal to the charge delivered to it (in Coulombs) divided by the capacitor's capacitance (in Farads). The charge delivered to the capacitor is equal to the integral of the current along the charging period. In this case, since the current is constant, we get a linear voltage charging profile of the capacitor.
 

Since the power delivered to the load at any moment, in this case a capacitor, is proportional to the output voltage multiplied by the output current, and since the output current is constant and the capacitor voltage increases linearly with time (starting from zero), we conclude that the output power the charging power supply delivers is initially zero, increasing linearly along the charging process.
 

For instance, let's suppose we are willing to deliver an energy of 3,000 Joules per second using a constant current charging power supply. At the beginning of the charging process the voltage across the capacitor is zero and accordingly, the power delivered by the charging power supply to the capacitor is zero, then it increases linearly until the charging process is complete (i.e. the desired voltage across the capacitor has been reached).
 

Since the capacitor voltage increases linearly along the charging period, so does the output power delivered by a constant current capacitor charging power supply (blue line), as shown in figure 1.
 

The energy in Joules delivered to the capacitor is equal to the area below the blue line in figure 1 below (vertical axis in Watts, horizontal axis in mSec.).
 

Since we require an average energy of 3,000 Joules per second (orange line), and the charging power delivered by the power supply starts from zero and increases linearly, the final, peak charging power the power supply delivers is 6,000 Joules per second, or 6,000W, as shown in figure 1 below.