Friday, December 21, 2012
Saturday, January 28, 2012
TECHNICAL PROJECTS
New Delhi Time
Forward converter
Indian Rupee Exchange Rate
Forward converter is another popular switched mode power supply (SMPS) circuit that is used for producing isolated and controlled dc voltage from the unregulated dc input supply. As in the case of fly-back converter the input dc supply is often derived after rectifying (and little filtering) of the utility ac voltage. The forward converter, when compared with the fly-back circuit, is generally more energy efficient and is used for applications requiring little higher power output (in the range of 100 watts to 200 watts). However the circuit topology, especially the output filtering circuit is not as simple as in the fly-back converter[3].
Fig. 7 shows the basic topology of the forward converter. It consists of a fast switching device ‘S’ along with its control circuitry, a transformer with its primary winding connected in series with switch ‘S’ to the input supply and a rectification and filtering circuit for the transformer secondary winding. The load is connected across the rectified output of the transformer-secondary.
The transformer used in the forward converter is desired to be an ideal transformer with no leakage fluxes, zero magnetizing current and no losses. The basic operation of the circuit is explained here assuming ideal circuit elements and later the non-ideal characteristics of the devices are taken care of by suitable modification in the circuit design. In fact, due to the presence of finite magnetizing current in a practical transformer, a tertiary winding needs to be introduced in the transformer and the circuit topology changes slightly.
Basic Principle of operation:
The circuit of Fig. 7 is basically a dc-to-dc buck converter with the addition of a transformer for output voltage isolation and scaling. When switch ‘S’ is turned on, input dc gets applied to the primary winding and simultaneously a scaled voltage appears across the transformer secondary. Dotted sides of both the windings are now having positive polarity. Diode ‘D1’, connected in series with the secondary winding gets forward biased and the scaled input voltage is applied to the low pass filter circuit preceding the load. The primary winding current enters through its dotted end while the secondary current comes out of the dotted side and their magnitudes are inversely proportional to their turns-ratio. Thus, as per the assumption of an ideal transformer, the net magnetizing ampere-turns of the transformer is zero and there is no energy stored in the transformer core. When switch ‘S’ is turned off, the primary as well as the secondary winding currents are suddenly brought down to zero. Current through the filter inductor and the load continues without any abrupt change. Diode ‘D2’ provides the freewheeling path for this current. The required emf to maintain continuity in filter-inductor current and to maintain the forward bias voltage across D2 comes from the filter inductor ‘L’ itself. During freewheeling the filter inductor current will be decaying as it flows against the output voltage (Vop), but the presence of relatively large filter capacitor ‘C’ still maintains the output voltage nearly constant. The ripple in the output voltage must be within the acceptable limits. The supply switching frequency is generally kept sufficiently high such that the next turn-on of the switch takes place before the filter inductor current decays significantly. Needless to say, that the magnitudes of filter inductor and capacitor are to be chosen appropriately.
The idea behind keeping filter inductor current nearly constant is to relieve the output capacitor from supplying large ripple current. [As per the circuit topology of Fig.7, the inductor and the capacitor together share the load-current drawn from the output. Under steady state condition, mean dc current supplied by the capacitor is zero but capacitor still supplies ripple current. For maintaining constant load current, the inductor and capacitor current-ripples must be equal in magnitude but opposite in sense. Capacitors with higher ripple current rating are required to have much less equivalent series resistor (ESR) and equivalent series inductor (ESL) and as such they are bulkier and costlier. Also, the ESR and ESL of a practical capacitor causes ripple in its dc output voltage due to flow of ripple current through these series impedances. Since the output voltage is drawn from capacitor terminal the ripple in output voltage will be less if the capacitor is made to carry less ripple current.]
For better understanding of the steady-state behavior of the converter, the circuit’s operation is divided in two different modes, mode-1 and mode-2. Mode-1 corresponds to the ‘on’ duration of the switch and mode-2 corresponds to its ‘off’ duration.
The following simplifying assumptions are made before proceeding to the detailed mode-wise analysis of the circuit:
• ON state voltage drops of switches and diodes are neglected. Similarly, leakage currents through the off state devices is assumed zero. The switching-on and switching-off times of the switch and diodes are neglected.
• The transformer used in the circuit is assumed to be ideal requiring no magnetizing current, having no leakage inductance and no losses.
• The filter circuit elements like, inductors and capacitors are assumed loss-less.
• For the simplified steady-state analysis of the circuit the switch duty ratio (δ), as defined in the previous chapters is assumed constant.
• The input and output dc voltages are assumed to be constant and ripple-free. Current through the filter inductor (L) is assumed to be continuous.
Mode-1 of Circuit Operation:
Mode-1 of circuit starts after switch ‘S’ (as shown in Fig.7) is turned ON. This connects the input voltage, Edc, to the primary winding. Both primary and secondary windings start conducting simultaneously with the turning on of the switch. The primary and secondary winding currents and voltages are related to their turns-ratio (NP / NS), as in an ideal transformer.
Fig.8 (a) shows, in bold lines, the current carrying path of the circuit and Fig.23.2 (b) shows the functional equivalent circuit of mode-1. As switch ‘S’ closes, diode D1 in the secondary circuit gets forward biased and the input voltage, scaled by the transformer turns ratio, gets applied to the secondary circuit. Diode D2 does not conduct during mode-1, as it remains reverse biased.
As can be seen, the output circuit consisting of L-C filter and the load gets a voltage equal to NS/NP Edc during mode-1. This voltage is shown across points ‘P’ and ‘N’ in Fig. 8(b) and it is the maximum achievable dc voltage across the load, corresponding to δ = 1. Mode-1 can be called as powering mode during which input power is transferred to the load. Mode-2, to be called as freewheeling mode, starts with turning off of the switch ‘S’.
Mode-2 of Circuit Operation:
As soon as switch ‘S’ is turned off, the primary and the secondary winding currents of the transformer fall to zero. However, the secondary side filter inductor maintains a continuous current through the freewheeling diode ‘D2’. Diode ‘D1’ remains off during this mode and isolates the output section of the circuit from the transformer and the input.
Fig. 9(a) shows the current carrying portion of the circuit in bold line and Fig.9(b) shows the equivalent circuit active during mode-2. Points ‘P’ and ‘N’ of the equivalent circuit are effectively shorted due to conduction of diode ‘D2’. The inductor current continues to flow through the parallel combination of the load and the output capacitor. During mode-2, there is no power flow from source to load but still the load voltage is maintained nearly constant by the large output capacitor ‘C’. The charged capacitor and the inductor provide continuity in load voltage. However since there is no input power during mode-2, the stored energy of the filter inductor and capacitor will be slowly dissipating in the load and hence during this mode the magnitudes of inductor current and the capacitor voltage will be falling slightly. In order to keep the load voltage magnitude within required tolerance band, the converter-switch ‘S’ is turned on again to end the freewheeling mode and start the next powering mode (mode-1). Under steady state, loss in inductor current and capacitor voltage in mode-2 is exactly made up in mode-1. It may not be difficult to see that to maintain load voltage within the desired tolerance band the filter inductor and capacitor magnitudes should be sufficiently large. However, in order to keep the filter cost and its physical size small these elements should not be unnecessarily too large. Also, for faster dynamic control over the output voltage the filter elements should not be too large. [It may be pointed out here that the filter inductor, capacitor, transformer and the heat sinks for the switching devices together account for nearly 90% of the power supply weight and volume.]
Switch limitations have been ignored in the simplified analysis presented here. As mentioned earlier, the switch and the diodes have been assumed to be ideal, with no losses and zero switching time. Control over switch duty ratio, which is the ratio of ON time to (ON + OFF) time, provides the control over the output voltage ‘VO’.
ARUN KUMAR MOHALLIK
ELECTRICAL & ELECTRONICS ENGINEER
Seeking a challenging position that offers me generous opportunities to explore & outshine in the field of while accomplishing personal as well as organizational goals in Electrical/ Electronics/ Telecommunication industry.
PROFILE
§ Academically astute individual with Diploma in Electrical & Electronics Engineering, possess deep passion and high energy level to pursue a successful career by blending skills & knowledge using creative instincts to attain goals of the organization.
§ Self motivated, hard working and goal-oriented with a high degree of flexibility, creativity, resourcefulness, commitment and optimism.
§ Conceptually strong with an innovative and analytical approach to the work with an eye for detail. Enriched with the ability to learn new concepts & technology within a short span of time.
§ Possess the ability to work comfortably in diverse teams and challenging environment with people from diverse backgrounds and experiences. Target and achievement oriented with an ability to take up challenges and perform in changing work environs.
§ Outstanding communication skills, verbal as well as written coupled with good presentation skills with the ability to perform above expectations. Multilingual with proficiency in English, Hindi & Oriya.
Subscribe to:
Posts (Atom)