The aim behind the design of the IC TPS92310 (from TEXAS INSTRUMENTS) is to offer a constant current line and load regulation to the load through a primary side sensing flyback inductor, which operates in the critical conduction mode, and eliminates the need of the traditional opto coupler based secondary side feedback control.
The proposed design employs a non-isolated single inductor smps design and thus removes the obligatory transformers, making the design much compact and involving less BOM, yet meets the standard performance criteria of an LED driver specifications.
The design also includes a PFC stage for ensuring a cleaner output and satisfy the modern PFC IEC 61000-3-2 rules
The following explanation provides us with the operating principle of the proposed 7 watt LED driver SMPS circuit:
1) The LED controller chip TPS92314A includes an advanced constant ON-time control feature for ensuring a high power factor at the input, and quasi-resonant switching for guarantying greater efficiency and minimum EMI emission.
2) The design facilitates load power regulation through the stored energy of an inductor configured in the form of a high-side buck converter .
3) The inclusion of a diode/capacitor at the output additionally regulates the DC content, without depending on any extra auxiliary winding which is commonly seen in traditional isolated forms of SMPS designs...here this is eliminated causing the unit to become very compact, highly efficient and cost effective.
4) The figure shows a standard full bridge rectifier network at the input for converting the alternating input current into a single positive AC bus.
The pulsating sine voltage here faithfully follows the pulsating sine current. due to the presence of a 100nF capacitor immediately after the bridge rectifier, and this helps in maintaining a high power factor response.
5) The above processed supply is fed to the drain of a mosfet which is configured as a high side switching device, having its source hooked up with D8 freewheeling diode along with inductor L3 and output capacitor C5.
6) In the figure the IC input side of the IC could be seen referenced to a switching junction SW, which makes sure that the IC does not switch ON until the processed AC has a potential higher than the connected LED's forward voltage value, and also for so along as the input is not drawing any current. This parameter causes a delay factor during power switch, and can be calculated through the following expression:
Δ T = Sine (inverse)VLED / √2 xVac
During the critical conduction mode periods of the IC TPS92314, the peak current from the inductor becomes two times more than the input peak current.
The inductor value for this 7 watt LED driver SMPS circuit can be calculated using the following formula:
L = [1.41 x Vac - VLED] x Ton / ΔIpeak
Due to the fact that this IC involves a critical conduction mode operation implies that the every subsequent ON periods is initiated only once the current within the inductor has ramped down to almost zero.
A feedback voltage in the form of VLED is applied back to the IC which acts like a supply voltage for the IC, because VLED can be seen linked with the input side bridge network ground. This particular implementation allows the design to comfortably work with only a single non-isolated inductor and gets rid of the complex extra biasing winding.
This makes this 7 watt non isolated SMPS LED driver circuit extremely compact, durable, efficient and very long lasting and also compliant with the present SMPS laws.
The design can be adapted for all power LEDs ranging from 1 watt to 7 watt.
The main specifications of the driver circuit can be witnessed in the following table: